Composite lithium metal anodes for lithium batteries with reduced volumetric fluctuation during cycling and dendrite suppression

ABSTRACT

A lithium battery includes a cathode, a composite lithium metal anode, and an electrolyte in contact with the cathode and the composite lithium metal anode. The composite lithium metal anode includes a porous matrix and lithium metal disposed within the porous matrix.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 15/348,884,filed Nov. 10, 2016, which claims the benefit of U.S. ProvisionalApplication No. 62/253,980, filed Nov. 11, 2015, the content of which isincorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contractDE-EE0006828 awarded by the Department of Energy. The Government hascertain rights in the invention.

BACKGROUND

Lithium (Li) metal, due to its high theoretical capacity, lowelectrochemical potential, and light-weight nature, is widely recognizedto be the most prominent candidate for the next-generationhigh-energy-density Li battery anodes. Nevertheless, the dendriticdeposition nature as well as the virtually infinite relative dimensionchange during cycling severely impede its practical applications.

It is against this background that a need arose to develop theembodiments described in this disclosure.

SUMMARY

One aspect of this disclosure relates to a lithium battery. In someembodiments, the lithium battery includes a cathode, a composite lithiummetal anode, and an electrolyte in contact with the cathode and thecomposite lithium metal anode. The composite lithium metal anodeincludes a porous matrix and lithium metal disposed within the porousmatrix.

Another aspect of this disclosure relates to a method of manufacturing acomposite lithium metal anode. In some embodiments, the method includesproviding a porous matrix, providing liquefied lithium metal, andinfusing the liquefied lithium metal into the porous matrix.

Other aspects and embodiments of this disclosure are also contemplated.The foregoing summary and the following detailed description are notmeant to restrict this disclosure to any particular embodiment but aremerely meant to describe some embodiments of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of some embodimentsof this disclosure, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1. Schematic of a battery.

FIG. 2. Fabrication of layered Li-reduced graphene oxide (rGO) compositefilm a, Schematic of material design and the consequent syntheticprocedures from graphene oxide (GO) film (left) to sparked rGO film(middle) and to layered Li-rGO composite film (right). Correspondingdigital camera images of the GO film (b), sparked rGO film (c) andlayered Li-rGO composite film (d) are shown accordingly.

FIG. 3. Characterization of materials. Low-magnification (a) andmagnified (b) scanning electron microscope (SEM) images of the pristineGO film prepared by vacuum filtration. The corresponding SEM images ofsparked rGO film and layered Li-rGO film are shown in c,d and e,f,respectively. g, Fourier transform infrared spectroscopy (FTIR) spectraof pristine GO film (top) and sparked rGO film (bottom). h,iDeconvoluted X-ray photoelectron spectroscopy (XPS) spectra of C 1s (h)and O 1s (i) before (top) and after (bottom) spark reaction. j, Alayered Li-rGO strip coiled on a glass rod showing good flexibility. k,A layered Li-rGO strip twisted by two tweezers showing goodtwistability.

FIG. 4. Li deposition behavior and in situ characterization. Crosssection SEM images of the Li-rGO electrode before (a) and after (b) Listripping, and after 1 stripping/plating cycle (c). d,e SEM images ofthe bare Li foil surface after 10 galvanostatic cycles. f,g SEM imagesof the Li-rGO surface after 10 galvanostatic cycles. The current densityof the cells shown in d-g was fixed at about 1 mA cm⁻² while thecapacity was set at about 1 mAh cm⁻². h,i SEM image of the Li-rGOelectrode surface after 10 galvanostatic cycles with a higher currentdensity of about 3 mA cm⁻². j, Schematic of the in situ transmissionelectron microscope (TEM) cell configuration. k, Time-lapse imagesshowing the side view of rGO film during Li deposition process. l, Topview of rGO film at initial (top) and final (bottom) stage of Lideposition. m, Electron energy loss spectroscopy (EELS) spectra of rGOfilm before (bottom) and after (top) Li deposition. The signals werecollected at the positions marked in k at the initial (top dot) andfinal (bottom dot) stages, respectively.

FIG. 5. Electrochemical characterization of Li-rGO electrode. a,Galvanostatic cycling of symmetric Li-rGO electrode and bare Li foil inthe first 800,000 seconds. The current density was fixed at about 1 mAcm⁻² with stripping/plating capacity of about 1 mAh cm⁻². The detailedvoltage profiles were further displayed as the insets on the top (fromleft to right, shown are the 1st, 10th, 50th, and 100th cycles,respectively). The y axis scale of the inset figures is shown on theleft. b, Corresponding higher current density (about 3 mA cm⁻²) cyclingof symmetric Li-rGO electrode and bare Li foil in the first 250,000seconds with the detailed voltage profiles shown as the insets. c,Nyquist plots of the symmetric cells after 10 galvanostatic cycles. d,Full Li stripping curve of the Li-rGO electrode to about 1 V versusLi⁺/Li, which shows a specific capacity about 3390 mAh g⁻¹.

FIG. 6. Time-lapse images of the spark reaction visualizing the detailedphenomenon of the reaction within about 100 milliseconds. The images ofthe reaction at different time of 0 ms (a), 20 ms (b), 40 ms (c), 60 ms(d), 80 ms (e), and 100 ms (f) are shown successively. The arrow in ashows the initial contact point between GO and molten Li, where thereaction initiated. The flame shown in the images illustrates thepossible H₂ formation under the strong reduction condition in thepresence of molten Li and its combustion reaction with the trace amountof oxygen in the glove box. This can be one of the reasons for theinterlayer expansion of GO.

FIG. 7. Time-lapse images (a, 0 s; b, 5 s; c, 12 s; d, 20 s; e, 45 s) ofLi infusion process into the sparked-rGO film. The edge of thesparked-rGO film was put in contact with the molten Li. Rapid Liinfusion can be observed where it took less than about 1 minute for Lito spread across the whole sparked-rGO surface.

FIG. 8. Surface wetting of molten Li on different carbon materials,including carbon nanotube (CNT) film (a,f), carbon fiber paper (b,g),mesoporous carbon coated on Cu foil (c,h), electrospun carbon nanofiber(d,i) and sparked-rGO film (e,j). For sparked-rGO film, the molten Liwas rapidly infused into the matrix with good wettability. In contrast,the other carbon materials showed relatively large contact angle,indicating relatively poor Li surface wettability.

FIG. 9. First-principles calculations showing the binding energy betweenLi and bare graphene surface (a), carbonyl (C═O) groups (b), alkoxygroups (C—O) (c), and epoxyl (C—O—C) groups (d). The carbonyl and alkoxygroups show much stronger interaction with Li compared to bare graphenesurface.

FIG. 10. Schematic showing the effect of capillary force with differentsurface ‘lithiophilicity’ (‘lithiophobic’—left, ‘lithiophilic’—middle &right) and different interlayer gap dimension (‘larger interlayerdimension’—middle, ‘nanoscale interlayer dimension’—right). Thecapillary force on lyophobic surface will lower the liquid level whilethe lyophilic surface will lift the liquid level. The height of theliquid level is inversely proportional to the diameter, which meanssmaller spacing with lyophilic surface will give rise to higher liquidlevel.

FIG. 11. N₂ adsorption-desorption isotherms of the pristine GO film(bottom) and the sparked rGO film (top), from which theBrunauer-Emmett-Teller (BET) surface area was calculated to be about 8.0m² g⁻¹ and about 394.3 m² g⁻¹, respectively.

FIG. 12. XPS survey spectra of pristine GO (top) and sparked rGO(bottom). After spark reaction, significantly increased CIO ratio can beobserved, which indicates the removal of O-containing species and thereduction of GO in the spark process.

FIG. 13. Raman spectra of pristine GO (top) and sparked rGO (bottom)films. The sparked rGO showed lower D/G band ratio.

FIG. 14. X-ray diffraction (XRD) spectra of pristine GO film, sparkedrGO and Li-rGO composite. Pristine GO showed a sharp peak at 2θ of about11°, which is indicates a highly oxidized graphite with remarkablyincreased interlayer spacing (d of about 0.8 nm). The peak at 2θ ofabout 11° disappeared for sparked rGO, indicating the partial reductionof GO. A sharp peak corresponding to metallic Li (110) can be observedfor Li-rGO, indicating the successful infusion of Li into the rGOmatrix.

FIG. 15. SEM images of the Li-rGO electrodes with different thickness ofabout 50 μm (a,d), about 80 μm (b,e), and about 200 μm (c,f). Themagnified SEM images shown in d-f indicate consistent layered structurewith similar spacing despite the electrode thickness.

FIG. 16. Low-magnification (a) and magnified (b) SEM images of the topsurface of layered Li-rGO electrode after 10 galvanostatic cycles withcurrent density of about 5 mA cm⁻². The stripping/plating capacity wasfixed at about 1 mAh cm⁻². The images show relatively flat surface,although some Li can be observed on the top surface (b).

FIG. 17. Ex situ SEM characterization on the thickness change before(a), after (b) Li stripping and after one stripping/plating cycle (c).After Li stripping, a reduced thickness decrease of about 20% can beobserved. And after plating Li back, the thickness is similar to theoriginal state.

FIG. 18. Galvanostatic cycling of Li foil and Li-rGO film in symmetriccell configuration at the current density of about 2 mA cm⁻². Thestripping/plating capacity was fixed at about 1 mAh cm⁻². The detailedvoltage profiles of the 1st, 10th, 50th, and 100th cycles were furthershown in the inset figures with scale of y axis shown on the left.

FIG. 19. Voltage profiles of Li-rGO (left column) and Li foil (rightcolumn) symmetric cells at different cycles varied from the 1st to the100th cycle. Profiles at different current densities of about 1 mA cm⁻²(a,b), about 2 mA cm⁻² (c,d) and about 3 mA cm⁻² (e,f) were chosen forcomparison.

FIG. 20. Nyquist plots of the symmetric cells of Li foil (top) andlayered Li-rGO (bottom) before electrochemical cycling. Li foil showedconsiderably larger interfacial resistance compared to the layeredLi-rGO counterpart.

FIG. 21. a, Galvanostatic cycling of Li foil and Li-rGO film symmetriccells in ether-based electrolyte (about 1 M LiTFSI in about 1:1, v/vDOL/DME with about 1% LiNO₃). Li-rGO electrode showed much loweroverpotential as well as more stable cycling stability compared to theLi foil counterpart. The curves of 800,000-1,000,000 seconds (left dashrectangle) and 2,800,000-3,000,000 seconds (right dash rectangle) wereenlarged and shown in b and c, respectively. The Li-rGO electrodeexhibited extremely stable cycling performance in the DOL/DMEelectrolyte, with stable cycling of >450 cycles as shown in a.

FIG. 22. Schematic of lithium stripping/plating processes on differentlithium electrodes. Schematic illustration of lithium stripping/platingprocesses on Li metal foil (a) with limited activated top surface area,and those on Li⁺ conductive matrix (b) with highly increased activatedsurface area.

FIG. 23. Synthetic procedures and stripping/plating behavior ofas-obtained electrode. (a) Synthetic procedures of Li—Li₂₂Si₅—Li₂Onanocomposite electrodes through thermal overlithiation and mechanicalpressing process. Digital image (b) and corresponding SEM image (e) ofpristine Li⁺ conductive nanocomposite Li metal electrode (LCNE). Digitalimage (c) and corresponding SEM image (f) of LCNE after stripping about8 mAh cm⁻² of Li with current density of about 2 mA cm⁻². Digital image(d) and corresponding SEM image (g) of LCNE after stripping and platingabout 8 mAh cm⁻² of Li with same stripping/plating current density ofabout 2 mA cm⁻². Cross section SEM images of pristine electrode (h),electrode after stripping about 8 mAh cm⁻² of Li (i), and electrodeafter stripping and plating back about 8 mAh cm⁻² of Li (j). The threeelectrodes show substantially the same thickness indicating highlyconstant electrode volume during cycling.

FIG. 24. Characterization on morphology of Li deposited on different Limetal electrodes after 20 cycles. (a) Schematic illustration of SEMobservation of Li deposited on Li metal foil and corresponding lowmagnification (c) and magnified (e) SEM images showing dendritic growthof Li. (b) Schematic illustration of SEM observation of Li deposited onLCNE and corresponding low magnification (d) and magnified (f) SEMimages showing no observable dendritic Li growth. The current density isfixed at about 1 mA cm⁻² for both Li stripping and plating processes.The stripping/plating capacity is about 1 mAh cm⁻².

FIG. 25. Electrochemical characteristics of Li stripping/plating andtheir mechanisms. (a) Typical stripping/plating voltage profile of theLi foil and LCNE of the 1st galvanostatic cycle. (b) Nyquist plots ofsymmetric cells with Li foil and LCNE after 10 galvanostatic cycles. (c)Schematic shows the initial stripping/plating barrier (corresponding tobarrier A in (a)), plating barrier (corresponding to barrier B in (a))and further stripping barrier (corresponding to barrier C in (a)). (d)Voltage profile of Li foil symmetric cell and LCNE symmetric cell at the1st, 2nd, 10th, and 100th cycle. The current density and capacity of thecycling are fixed to be about 1 mA cm⁻² and about 1 mAh cm⁻²,respectively.

FIG. 26. Electrochemical performance of LCNE symmetric cell. The voltageprofiles of Li foil symmetric cells and LCNE symmetric cells at variouscurrent densities of about 1 mA cm⁻² (a), about 3 mA cm⁻² (b), and about5 mA cm⁻² (c). Stripping/plating capacity is fixed at about 1 mAh cm⁻².(d) Voltage profile of Li foil symmetric cell and that of LCNE symmetriccell at different various rates from about 0.5 mA cm⁻² to about 5 mAcm⁻².

FIG. 27. Rate capability of Li—S batteries with different Li electrodes.Voltage profile of Li—S batteries with LCNE (a) and Li foil (b) asnegative electrodes. Mass loading of S is fixed at about 2 mg cm⁻².C-rate is varied from about 0.2 C (about 0.669 mA cm⁻²) to about 2 C(about 6.69 mA cm⁻²). (c) Capacity retention of Li—S batteries atdifferent C-rate with LCNE and Li foil as negative electrodes.

FIG. 28. Various thickness of LCNE produced by mechanical pressing. SEMimages show the LCNE fabricated from overlithiated nanocomposite powderswith various thickness of about 120 μm (a, e), about 180 μm (b, f),about 300 μm (c, g), and about 500 μm (d, h).

FIG. 29. Phase characterization of LCNE at different stages. (a)Charging voltage profile of LCNE after fully delithiated to about 1 Vversus Li⁺/Li. (b) XRD spectrums of different delithiated stages (A:pristine LCNE; B: LCNE charged to about 0.15 V; C: LCNE charged to about1 V) as denoted by arrows in (a). At pristine stage A, evident XRD peaksof Li, Li₂O and Li₂₂Si₅ are shown. When the LCNE was charged to about0.15 V (B), Li peak disappeared while the Li₂O and Li₂₂Si₅ peaks stillmaintained. After fully charging the LCNE to about 1 V, most of thepeaks of Li₂₂Si₅ also disappeared, and just the major peak was stillobservable, which indicates that most of Li₂₂Si₅ was delithiated.

FIG. 30. Rigid matrix shown by substantially fully stripping all Li fromthe electrode. The figures show the pristine electrode (left) and theelectrode after substantially fully stripping all Li (right). Afreestanding rigid matrix is retained after substantially fullystripping all Li without structure collapse. This indicates that thematrix obtained by overlithiation affords an interconnected nature tosupport a stable dimension.

FIG. 31. Detailed comparison on voltage profiles at various currentdensities in carbonate electrolyte system. Single-cycle voltage profilesof LCNEs (a) and Li foil electrodes (b) as current density varied fromabout 0.5 mA cm⁻² to about 5 mA cm⁻². The capacity is fixed at about 1mAh cm⁻².

FIG. 32. Voltage profiles comparison and the stability after cycling incarbonate electrolyte system. (a) Voltage profiles of the 2nd cycle andthe 10th cycle of LCNEs (solid lines) and Li metal foils (dash lines).(b) The evolution of voltage profiles after cycling. The 2nd, 10th, and50th cycles are shown here. The current density is fixed at about 1 mAcm⁻².

FIG. 33. Impedance spectroscopy study before cycling. Nyquist plot ofimpedance measurement before cycles (a) and the corresponding enlargedplot (b, rectangle in a). This shows significant different interfacialresistance between Li foils and LCNEs.

FIG. 34. Electrochemical performance of LCNEs symmetric cell inether-based electrolyte. The voltage profiles of Li foil symmetric cellsand LCNE symmetric cells in about 1:1 v/v DOL/DME with about 1% LiNO₃are shown at various current densities of about 1 mA cm⁻² (a), and about5 mA cm⁻² (b). Stripping/plating capacity is fixed at about 1 mAh cm⁻².(c) Voltage profiles of the 2nd cycle and the 10th cycle of LCNEs (solidlines) and Li metal foils (dashed lines). (d) The evolution of voltageprofiles after cycling. The 2nd, 10th, 50th, and 100th cycles are shownhere. For (c) and (d), the current density is fixed at about 1 mA cm⁻².

FIG. 35. The pristine SiO powders (a) and the corresponding powder afterabout 5 hrs of high energy mechanical milling (b). The color of thepowders becomes lighter after mechanical milling which indicates thatthe size of the SiO becomes much smaller.

FIG. 36. The as-obtained Li—Li₂₂Si₅—Li₂O nanocomposite powders byoverlithiation.

FIG. 37. Schematic of the fabrication of the Li-coated polyimide (PI)matrix. Electrospun PI was coated with a layer of ZnO via atomic layerdeposition (ALD) to form core-shell PI—ZnO. The ZnO coating renders thematrix “lithiophilic” such that molten Li can readily infuse into thematrix. The final structure of the electrode is Li coated onto a porous,non-conducting polymeric matrix.

FIG. 38. Characterization of the Li-coated PI electrode. SEM images ofthe electrospun PI fibers (a) before and (b) after ZnO coating. (c)Cross-sectional SEM image of the core-shell PI—ZnO, where the conformalZnO coating can be observed from the contrast of the fibercross-sections. (d) Scanning transmission electron microscope (STEM)image of a single core-shell PI—ZnO fiber and the corresponding energydispersive X-ray (EDX) elemental mapping of C and Zn distribution. (e)SEM image of the Li-coated PI matrix, showing the porous nature of theLi electrode. (f) XRD spectra of the pristine PI, the core-shell PI—ZnOand the Li-coated PI—ZnO matrix, where the Li-coated PI—ZnO exhibitedthe signals of LiZn alloy, Li₂O and metallic Li. (g) FTIR spectra of thepristine PI, the core-shell PI—ZnO and the Li-coated PI—ZnO matrix (Liwas scraped away to expose the underlying matrix in order to obtain thesignal). The characteristic peaks of PI remained after ZnO and Licoating, indicating the stability of the polymeric matrix.

FIG. 39. SEM images and the corresponding schematics showing thewell-confined stripping/plating behavior of the Li-coated PI matrix. Topview SEM images of (a) the exposed top fibers of the Li-coated PIelectrode after stripping away about 5 mAh/cm² Li, (b) exposed topfibers partially filled with Li when plating about 3 mAh/cm² Li back and(c) substantially completely filled PI matrix after plating anadditional about 2 mAh/cm² Li back. (Current density at about 1 mA/cm²,in EC/DEC). The polymeric matrix ensures Li is dissolved and depositedfrom the underlying conductive Li substrate and, as a result, Li iseffectively confined into the matrix. The top-right schematicillustrates the alternative undesirable Li stripping/plating behaviorwhere after striping, Li nucleate on the top surface, leading to volumechange and dendrites extending out of the matrix.

FIG. 40. Morphology characterization of the Li electrodes after cyclingat different current densities. Top view SEM images of the Li-coated PImatrix after 10 cycles of stripping/plating in EC/DEC at a currentdensity of (a) about 1 mA/cm², (b) about 3 mA/cm² and (c) about 5mA/cm². (d, e) Top view SEM images of the bare Li electrode after 10cycles of stripping/plating in EC/DEC at a current density of about 1mA/cm² with large amounts of mossy Li dendrites. Cross-sectional SEMimages of the Li-coated PI matrix (f) before and (g) after substantiallycomplete Li stripping, from which no significant volume change can beseen (note that the uniform fibrous features in a-c are not dendritesbut the fibrous matrix, which are distinctly different from thenon-uniform, random-sized mossy Li dendrites in d and e).

FIG. 41. Electrochemical characterization of the Li electrodes in EC/DECelectrolyte. Comparison of the cycling stability of the Li-coated PImatrix and the bare Li electrode at a current density of (a) about 1mA/cm², (b) about 3 mA/cm² and (c) about 5 mA/cm². (d) Rate performanceof the Li-coated PI matrix and the bare Li electrode. (e) Nyquist plotof the impedance spectra of the symmetrical Li-coated PI matrix and thebare Li cell after 10 cycles at a current density of about 1 mA/cm².Voltage profiles of (f) Li-coated PI matrix and (g) bare Li electrode atdifferent current densities after 10 cycles. The amount of Li cycled wasabout 1 mAh/cm² in all cases.

FIG. 42. Thermogravimetric analysis (TGA) curve of the electrospun PImatrix in air atmosphere.

FIG. 43. Image of a bare PI matrix and a core-shell PI—ZnO matrix inmolten Li indicating the difference in Li wettability.

FIG. 44. Cross-sectional SEM image of the core-shell PI—ZnO fiber.

FIG. 45. EDX line scan of the PI fiber after ZnO ALD resolving thecore-shell structure of the fiber. (C signal was originated from the PIpolymeric core and the Zn signal was originated from the ZnO shell).

FIG. 46. Cross-sectional SEM image of the Li-coated PI matrix.

FIG. 47. Cyclic voltammetry (CV) scan of pristine PI at a scan rate ofabout 1 mV/s.

FIG. 48. A typical Li stripping curve of the Li-coated PI matrix with astripping current density of about 0.25 mA/cm².

FIG. 49. Cross-sectional SEM images of the Li-coated PI matrix withdifferent thicknesses.

FIG. 50. Comparison of the cycling stability of the Li-coated PI matrixand the bare Li electrode at a current density of (a) about 1 mA/cm²,(b) about 3 mA/cm² and (c) about 5 mA/cm² in DOL/DME electrolyte. Theamount of Li cycled was about 1 mAh/cm². Necking of the voltage profileof the bare Li (overpotential first decreases and then increases) can beobserved in (c) due to dendrite growth during cycling, which reduces theeffective current density, leading to reduced overpotential.

FIG. 51. Image of a Li-coated PI matrix and a bare Li electrode withabout 10 μl electrolyte. The Li-coated PI matrix can uptake a largeamount of electrolyte due to its porous nature.

FIG. 52. Nyquist plot of the impedance spectra of the symmetricalLi-coated PI matrix and the bare Li cell after 10 cycles in DOL/DME at acurrent density of about 1 mA/cm².

FIG. 53. Voltage profiles of Li-coated PI matrix and bare Li electrodeat a current density of (a) about 1 mA/cm², (b) about 3 mA/cm² and (c)about 5 mA/cm² after 5 cycles. The amount of Li cycled was about 1mAh/cm² in all cases.

FIG. 54. Selected overpotential profiles of the Li-coated PI electrodeand the bare Li electrode in EC/DEC at a current density of about 1mA/cm². The Li-coated electrode exhibited a flat stripping/platingprofile while that of the bare Li electrode showed large “overpotentialbumps”. The fluctuation attenuated in later cycles due to the formationof Li dendrites that roughened the surface and reduced the effectivecurrent density.

FIG. 55. Schematic and optical images of Li encapsulation bymelt-infusion. (A) Schematic illustration of the design of a Li—scaffoldcomposite. (B) Li wetting property of various porous materials with andwithout the Si coating. (C) Time-lapse images of Li melt-infusionprocess for lithiophilic and lithiophobic materials.

FIG. 56. Microscopic characterizations of the bare carbon fiber andSi-coated carbon fiber. (A) TEM image of a single uncoated carbon fiber.(B and C) TEM image (B) of a single Si-coated carbon fiber and itszoom-in image (C). (D, Inset) TEM-EDS linear scan profile obtained alongthe line on a single fiber shown in the inset. The TEM-EDS linear scanexhibits the different material compositions of inner and outer regionof the fiber. (E-J) SEM-EDS characterization. Top-view SEM image (E) andcross-section SEM image (H) of the fiber layer. The correspondingelemental mapping for silicon (F and I) and carbon (G and J). (A-C)Scale bar, 100 nm. (E) Scale bar, 500 nm. (H) Scale bar, 50 μm.

FIG. 57. SEM and XRD study of the Li/C composite material. Opticalimages and SEM characterizations of the modified carbon fiber networkbefore (A, C and D) and after (B, E and F) Li infusion. The insets showcorresponding high-magnification images. (G) XRD pattern collected fromthe modified carbon fiber before and after Li infiltration. (C and E)Scale bar, 1 μm (D and F) Scale bar, 50 Insets, scale bar, 2 μm.

FIG. 58. Volume fluctuation of the electrode during different cyclingstages. Voltage profile of a typical Li-stripping-plating process forthe anode (A) and its corresponding thickness (B) at various stages ofcycling. #1 corresponds to the stage before cycling; #2 corresponds tothe stage after about 50% Li extraction (about 10 mAh/cm²); #3corresponds to the stage after Li plating back. (C-E) A sequence ofcross-sectional SEM images showing the huge volume fluctuation and shapechange of the bare Li metal anode in a typical Li-stripping-platingcycle. (F-H) A sequence of cross-sectional SEM images showing therelatively stable Li/C electrode. (I and J) Morphology study ofcross-sections of bare Li electrode (I) and Li/C electrode (J) after 30cycles at a current density of about 3 mA/cm² for about 1 mAh/cm². (J)Cross-sectional SEM image of Li/C electrode showing an unaltered shapeafter cycling. (C-J) Scale bar, 50 Insets, scale bar, 2 μm.

FIG. 59. Electrochemical performances of Li symmetrical cells and Li/Csymmetrical cells. (A-D) Typical voltage profiles ofLi-plating-stripping process with current density of about 0.5 mA/cm²for about 0.5 mAh/cm² (A), about 3 mA/cm² for about 0.5 mAh/cm² (B),about 0.5 mA/cm² for about 1 mAh/cm² (C) and about 3 mA/cm² for about 1mAh/cm² (D). (E) Impedance spectroscopy of Li/C and bare Li electrode.(F) Comparison of voltage hysteresis of the Li-plating/stripping processfor Li/C and Li electrode under various current rates. (G) Long-termcycling performance of Li/C and bare Li symmetrical cells with currentdensity of about 3 mA/cm² for a total of about 1 mAh/cm².

FIG. 60. Statistics of the carbon fiber diameter distribution. (A) SEMimage of pristine carbon nanofiber before cycling. (B) Fiber diameterdistribution showing an average size of 196 nm with a standard deviationof 34 nm based on a count number of 50.

FIG. 61. Current-Voltage characteristics of the carbon fiber. (A)Schematics of the current-voltage measurement experiment setup. Thethickness of the carbon fiber film is about 100-120 μm and the area isabout 1 cm². (B) Current-voltage curve for through-plane resistancemeasurement of the carbon fiber film at a scan rate of about 50 mV/s for3 times repetition.

FIG. 62. Sheet resistance of PAN-based carbon fiber film using a fourpoint meter based on 20 samples. The samples were cut into rectangles ofabout 1 cm×3 cm with a thickness of about 100-120 μm.

FIG. 63. Electrode thickness at 15 locations for the Li electrode andLi/C electrode cross-sections at different stages. (A) Electrodethickness of bare Li electrode at pristine stage, stripped stage andplated stage. (B) Electrode thickness of Li/C electrode at pristinestage, stripped stage and plated stage.

FIG. 64. Morphology study of Li electrode and Li/C electrode after 30cycles at a current density of about 3 mA/cm² for about 1 mAh/cm². (Aand B) Top-view SEM images of ramified growth of dendritic Li for bareLi electrode. (C and D) Top-view SEM images of Li/C electrode showing arelatively smooth surface. (A and C) Scale bar, 20 (B and D) Scale bar,5 μm.

DESCRIPTION

Rechargeable batteries based on Li metal anodes are desirable for nextgeneration high-energy-density and high-power electrochemical energystorage. Nevertheless, excessive dendritic growth, virtually infinitedimension change during repeated cycles together with limited poweroutput severely hinder practical applications. The ‘hostless’ nature ofthe Li stripping/plating process is the origin of many difficultiesassociated with the use of Li metal anodes. Here, some embodiments aredirected to the design of an anode in which a porous matrix or scaffoldis included as a “host” for metallic lithium, which together with theporous matrix form a composite lithium metal anode. Supported by amechanically stable or rigid matrix, reduced electrode-level dimensionalvariation during repeated cycling can be achieved. Moreover, the porousnature of the matrix can accommodate deposition of Li within the matrixand can afford a large activated surface area for Li deposition, whichyields enhanced uniformity of nucleation with little or no dendriteformation. Li batteries incorporating composite lithium metal anodes candemonstrate enhanced capacity retention, including under high-poweroperation. The improvement in electrochemical performance indicates theeffectiveness of the anode design for developing safe and stablemetallic Li anodes.

Some embodiments are directed to a lithium battery 100, which includes acathode 102, an anode 106, and an electrolyte 104 disposed between andin contact with the cathode 102 and the anode 106 (see FIG. 1). In someembodiments, the battery 100 is a lithium-ion battery, and the cathode102 includes a transition metal oxide, such as lithium cobalt oxide(LiCoO₂), lithium manganese oxide (LiMn₂O₄), lithium nickel manganesecobalt oxide (LiNi_(x)Mn_(y)Co_(z)O₂), or lithium iron phosphate(LiFePO₄). In some embodiments, the battery 100 is a lithium-sulfurbattery, and the cathode 102 includes sulfur. In some embodiments, thebattery 100 is a lithium-air battery, and the cathode 102 is a gascathode, such as formed of porous carbon or other porous material. Insome embodiments, the electrolyte 104 is a liquid electrolyte, such as acarbonate or an ether-based electrolyte, optionally including one ormore additives.

In some embodiments, the anode 106 is a composite lithium metal anode,and includes a porous matrix and metallic lithium disposed within poresor other open spaces within the matrix. Depending on the particularimplementation, the porous matrix can be electronically insulating,electronically conductive, or semiconducting; and can be ionicallyinsulating with respect to Li ions (Lit), or ionically conductive withrespect to Lit; or can have a combination of two or more of theforegoing characteristics. In the case of an electronically conductiveimplementation, the porous matrix can have a through-plane resistance ofabout 500 ohm or less, about 400 ohm or less, about 300 ohm or less,about 200 ohm or less, or about 100 ohm or less, and down to about 60ohm or less, or down to about 50 ohm or less, and a sheet resistance ofabout 20000 ohm/sq or less, about 15000 ohm/sq or less, about 10000ohm/sq or less, about 8000 ohm/sq or less, or about 6000 ohm/sq or less,and down to about 4000 ohm/sq or less, or down to about 2000 ohm/sq orless. In some embodiments, the porous matrix includes a fibrousmaterial, such as a network of fibers formed of carbon, polyimide oranother polymer, or another suitable material. For example, the porousmatrix can include carbon fibers (e.g., carbon nanofibers) or polymericfibers (e.g., polymeric nanofibers) having lateral dimensions (e.g.,diameters), or an average lateral dimension (e.g., an average diameter),in the nanometer (nm) range, such as in the range of about 10 nm toabout 1000 nm, about 50 nm to about 1000 nm, about 50 nm to about 500nm, about 100 nm to about 1000 nm, about 100 nm to about 500 nm, about200 nm to about 1000 nm, about 200 nm to about 500 nm, about 300 nm toabout 1000 nm, or about 300 nm to about 500 nm. In some embodiments, theporous matrix includes a layered material, such as layered reducedgraphene oxide or another suitable layered material. In someembodiments, the porous matrix includes a porous film, such as formed ofa Li⁺ conductive material or another suitable material. Other types ofporous matrices are contemplated, such as in the form of foams ormeshes.

Examples of suitable materials for the porous matrix include carbon orcarbon-based materials, such as carbon fibers (e.g., carbon nanofibers),polymeric fibers (e.g., polymeric nanofibers), layered reduced grapheneoxide, carbon nanotubes, and mesoporous carbon.

Additional examples of suitable materials for the porous matrix includeLi⁺ conductive materials, such as lithium-containing binary alloys ofthe formula Li_(x)M, where M is a Group 14 element, and x indicates theatomic ratio of Li to M and can be, for example, about 5:1 or less,about 4.5:1 or less, about 4:1 or less, about 3:1 or less, or about 2:1or less, and down to about 1:1, down to about 1:3, or down to about 1:6.In some embodiments, x is in the range of about 5:1 to about 4:1 orabout 4:1 to about 3:1. The Group 14 element can be selected from, forexample, carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead(Pb). In some embodiments, the porous matrix includes Li_(x)C. In someembodiments, the porous matrix includes Li_(x)Si. In some embodiments,the porous matrix includes Li_(x)Ge. In some embodiments, the porousmatrix includes Li_(x)Sn. In some embodiments, the porous matrixincludes nanoparticles that include Li_(x)M, such as having at least onedimension in the range of about 1 nm to about 1000 nm, about 500 nm orless, about 400 nm or less, about 300 nm or less, or about 200 nm orless, and down to about 100 nm or less, down to about 50 nm or less,down to about 20 nm or less, or down to about 10 nm or less. In someembodiments, the porous matrix further includes, in addition to Li_(x)M,a lithium-containing compound, such as an oxide, a fluoride, a sulfide,a selenide, a chloride, a bromide, a iodide, a phosphide, or a nitrideof lithium. In some embodiments, the porous matrix includes an oxide oflithium, such as Li₂O. In some embodiments, the porous matrix includesnanoparticles that include a lithium-containing compound, such as havingat least one dimension in the range of about 1 nm to about 1000 nm,about 500 nm or less, about 400 nm or less, about 300 nm or less, orabout 200 nm or less, and down to about 100 nm or less, down to about 50nm or less, down to about 20 nm or less, or down to about 10 nm or less,and where the nanoparticles including the lithium-containing compoundare interspersed with nanoparticles including Li_(x)M to form the porousmatrix. Further examples of suitable Li⁺ conductive materials includesolid-state electrolyte materials such as lithium phosphorus oxynitride(LiPON), Li₇La₃Zr₂O₁₂ (LLZO), Li₁₀GeP₂S₁₂ (LGPS), LLAO, and so forth.

In some embodiments, a characterization of the porous matrix is itsporosity, which is a measure of the extent of voids resulting from thepresence of pores or any other open spaces in the porous matrix. Aporosity can be represented as a ratio of a volume of voids relative toa total volume, namely between 0 and 1, or as a percentage between 0%and 100%. In some embodiments, the porous matrix can have a porositythat is at least about 0.1 and up to about 0.95 or more, and, moreparticularly, a porosity can be in the range of about 0.1 to about 0.9,about 0.2 to about 0.9, about 0.3 to about 0.9, about 0.4 to about 0.9,about 0.5 to about 0.9, about 0.5 to about 0.8, or about 0.6 to about0.8. Techniques for determining porosity include, for example,porosimetry and optical or scanning techniques.

In some embodiments, another characterization of the porous matrix isits surface area, such as its Brunauer-Emmett-Teller (BET) surface area,which can be at least about 9 m² g⁻¹, at least about 10 m² g⁻¹, at leastabout 13 m² g⁻¹, at least about 15 m² g⁻¹, at least about 20 m² g⁻¹, atleast about 50 m² g⁻¹, at least about 100 m² g⁻¹, at least about 200 m²g⁻¹, or at least about 300 m² g⁻¹, and up to about 400 m² g⁻¹ orgreater, or up to about 500 m² g⁻¹ or greater.

In some embodiments, another characterization of the porous matrix isits light-weight nature, such as having a density of about 3 g cm⁻³ orless, about 2.8 g cm⁻³ or less, about 2.5 g cm⁻³ or less, about 2.3 gcm⁻³ or less, about 2 g cm⁻³ or less, or about 1.8 g cm⁻³ or less, anddown to about 1.5 g cm⁻³ or less, down to about 1.3 g cm⁻³ or less, ordown to about 1 g cm⁻³ or less. In some embodiments, in at least onecycling state of the battery 100 (such as one in which the anode issubstantially fully plated with Li), the porous matrix can account forno more than about 50% of a total weight of the anode 106, such as nomore than about 40%, no more than about 35%, no more than about 30%, nomore than about 25%, no more than about 20%, no more than about 15%, nomore than about 10%, or no more than about 8%, and down to about 7% orless, or down to about 5% or less.

In some embodiments, the anode 106 omits a current collector, and iscomposed primarily of (e.g., in terms of weight percentage), consistsessentially of, or consists of the porous matrix and metallic lithium.

In some embodiments, lithium metal is included in the anode 106 as Lidomains (e.g., nano-sized Li domains) within pores or any other openspaces in the porous matrix. In some embodiments, the Li domains have atleast one dimension in the range of about 1 nm to about 1000 nm, such asabout 900 nm or less, about 800 nm or less, about 700 nm or less, about600 nm or less, about 500 nm or less, about 400 nm or less, about 300 nmor less, or about 200 nm or less, and down to about 100 nm or less, downto about 50 nm or less, down to about 20 nm or less, or down to about 10nm or less.

In some embodiments, the anode 106 is formed as a composite lithiummetal anode, by a manufacturing method including providing the porousmatrix, providing liquefied or molten Li metal (e.g., in a state at orabove the melting point of Li of about 180° C.), and infusing orinfiltrating the liquefied Li metal into the porous matrix. In someembodiments, the porous matrix has a high thermal stability up to atemperature above the melting point of Li, such as up to about 600° C.,up to about 550° C., up to about 500° C., up to about 450° C., up toabout 400° C., up to about 350° C., or up to about 300° C., asreflected, for example, through thermogravimetric analysis. In someembodiments, the porous matrix has a high electrochemical and chemicalstability with respect to Li, so as to reduce or avoid undesiredreactions during manufacturing or during Li stripping/plating processes.

In some embodiments, the porous matrix is intrinsically “lithiophilic”or is rendered or otherwise treated to become “lithiophilic”, so as tofacilitate infusing of lithium metal into the porous matrix.“Lithiophilicity” or “lithiophilic” nature of a material refers to anaffinity of the material towards lithium metal, such as in its liquefiedor molten state. In some embodiments, “lithiophilic” nature of amaterial can be characterized according to wettability of a solidsurface of the material by liquefied or molten Li metal. A measure ofwettability is a contact angle between the solid surface and a drop ofliquefied Li metal disposed on the surface, where the contact angle isthe angle at which the liquid-vapor interface intersects thesolid-liquid interface. As the tendency of the liquefied Li metal tospread over the solid surface increases, the contact angle decreases.Conversely, as the tendency of the liquefied Li metal to spread over thesolid surface decreases, the contact angle increases. Contact anglesless than 90° (low contact angles) typically indicate that wetting ofthe solid surface is favorable (high wetting), while contact anglesgreater than or equal 90° (high contact angles) typically indicate thatwetting of the surface is unfavorable (low wetting). In someembodiments, the porous matrix is or is rendered “lithiophilic” so as toform a contact angle with liquefied Li metal of less than 90°, such asabout 89° or less, about 87° or less, about 85° or less, about 80° orless, about 75° or less, about 70° or less, about 65° or less, about 60°or less, or about 50° or less, and down to about 30° or less, down toabout 20° or less, or down to about 10° or less.

In some embodiments, the porous matrix is formed of an intrinsically“lithiophilic” material, such as layered reduced graphene oxide, and isreadily infused with liquefied Li metal to form a composite lithiummetal anode. In some embodiments, the porous matrix is rendered orotherwise treated to become “lithiophilic”, such as by applying acoating of a “lithiophilic” material to an initial porous material toform the porous matrix that is rendered “lithiophilic”, by surfacefunctionalization with functional groups, such carbonyl (C═O), alkoxy(C—O), or epoxyl (C—O—C), which afford strong Li binding, or by removingsurface functional groups which afford weak Li binding. In someembodiments, the porous matrix is rendered “lithiophilic” by applying acoating of M, such as by chemical vapor deposition (CVD), and infusionof liquefied Li metal can yield a reaction that forms Li_(x)M andadditional Li metal, where M is a Group 14 element, and x indicates theatomic ratio of Li to M and can be, for example, about 5:1 or less,about 4.5:1 or less, about 4:1 or less, about 3:1 or less, or about 2:1or less, and down to about 1:1, down to about 1:3, or down to about 1:6.In some embodiments, x is in the range of about 5:1 to about 4:1 orabout 4:1 to about 3:1. The Group 14 element can be, for example,silicon (Si), and the reaction can yield Li_(x)Si and Li. In someembodiments, the porous matrix is rendered “lithiophilic” by applying acoating of a metal (M′)-containing compound, such as an oxide of M′ thatis applied by atomic layer deposition (ALD), and infusion of liquefiedLi metal can yield a reaction that forms Li_(y)M′, a lithium-containingcompound, such as an oxide of Li like Li₂O, and additional Li metal,where M′ is a metal different from Li, such as zinc (Zn) or othertransition metal, and y indicates the atomic ratio of Li to M′ and canbe, for example, about 3:1 or less, about 2:1 or less, or about 1:1 orless.

EXAMPLES

The following examples describe specific aspects of some embodiments ofthis disclosure to illustrate and provide a description for those ofordinary skill in the art. The examples should not be construed aslimiting this disclosure, as the examples merely provide specificmethodology useful in understanding and practicing some embodiments ofthis disclosure.

Example 1 Layered Reduced Graphene Oxide with Nanoscale Interlayer Gapsas Stable Host for Lithium Metal Anodes

Overview

Lithium (Li) ion batteries have gained commercial success as thedominating power source for portable electronics and electric vehicles.Li metal, due to its highest theoretical capacity (about 3860 mAh g⁻¹),lowest electrochemical potential (about −3.040 V versus StandardHydrogen Electrode), and light-weight nature (about 0.534 g cm⁻³), iswidely recognized to be the most prominent candidate for thenext-generation high-energy-density Li battery anodes. Nevertheless, thedendritic deposition nature as well as the virtually infinite relativedimension change during cycling severely impede its practicalapplications. Extensive efforts have been devoted to tackle the problemsof uneven Li deposition and unstable solid electrolyte interphase (SEI),while the electrode dimension stability is generally ignored. In thisexample, a ‘host’ for Li metal was introduced to solve the multifacetedproblems. Exploiting the merits of carbon-based materials, inclusive oflight-weight, high-surface-area and excellent electrochemical/mechanicalstability, this example sets forth the design of a layered reducedgraphene oxide (rGO) film with nanoscale interlayer gaps as the Li metal‘host’. In contrast to other carbon materials, rGO sheets exhibitedsurprisingly good surface Li affinity, and when coupled with nanoscalecapillary force, uniform and efficient infusion of molten Li into theinterlayer gaps can be realized. The as-obtained Li-rGO nanoscalelayered structure exhibited high surface area, high Li content, andexcellent flexibility, and also offered significantly reduced dimensionchange on the whole electrode level during cycling (about 20% versusvirtually infinite for Li foil). As a consequence, reduced electrodedimension fluctuation, dendrite suppression, more stableelectrode/electrolyte interface and high specific capacity (about 3390mAh g⁻¹) can be fulfilled simultaneously, which opens up promisingopportunities for stable Li anodes with outstanding electrochemicalproperties.

INTRODUCTION

Lithium (Li) metal is a particularly desired anode material for Libatteries. To realize applicable Li metal anodes, effort has been madeto understand the Li deposition mechanism and tackle the problems ofdendrite formation as well as unstable solid electrolyte interphase(SEI). The major strategies employed can be summarized into thefollowing three categories: (i) developing electrolyte additives for theformation of stable SEI; (ii) engineering high modulus solid electrolyteto prevent dendrite penetration; and (iii) designing electrochemicallyand mechanically stable artificial interface for SEI stabilization anddendrite suppression. All of the above-mentioned approaches have beendemonstrated to be efficacious to a certain extent for mitigating eitheror both the issues of dendritic Li growth and SEI collapse.Nevertheless, none of them addresses the relatively infinite electrodedimension change brought by the ‘hostless’ Li plating/stripping. Itshould be recognized that the stabilization of the electrode dimensionis also important for the practical applications of Li anodes. Withinfinite relative volume change, the resulting huge internal stressfluctuation and the floating electrolyte/electrode interface can damagethe cells and lead to potential safety hazards, and also bring abouttremendous engineering challenges implementing the battery into awell-defined cell space. More importantly, SEI stabilization is alsostrongly correlative to dimensional stability. The SEI may not be ableto accommodate the drastic variation of interfacial morphology and wouldthus face the danger of collapse during continuous cycling. Such SEIcollapse can in turn exacerbate the locally favorable Li dendrite growthand the continuous electrolyte decomposition. As a consequence, animproved approach capable of solving the multifaceted problems includingdimensional stability is desired for practical applications.

The virtually infinite dimension change of Li is originated from its‘hostless’ nature. To address the problem, rational design of a ‘host’for metallic Li is desired. Here, the host should satisfy severalcriteria. First, a mechanically stable scaffold with high porosity isdesired in order to host maximum amount of Li and maintain a stabledimension during electrochemical cycling. Secondly, a light weightmaterial with high surface area for Li nucleation is favorable. Highdensity scaffold can significantly reduce the weight percentage ofinfused Li and thus lower the specific capacity; while limited surfacearea may concentrate the local ion flux, resulting in uneven deposition.In addition, the host material should be electrochemically stable underthe highly reductive condition in the presence of Li. Otherwise, sidereactions with the host will continuously consume the Li source throughcycling, giving rise to low Coulombic efficiency (CE).

Considering these criteria, carbon-based materials are desired ‘host’candidates for Li metal. Carbon is among the lightest materialsavailable in the periodic table for the scaffold construction. It isalso noted that carbon materials including carbon nanotubes (CNTs),graphene, and mesoporous carbon have appealingly high surface area withexcellent mechanical strength. Moreover, carbon materials are generallystable under the redox environment within Li batteries.

However, to design the ‘host’ for Li metal, one extra criterion, namely,outstanding Li affinity (‘lithiophilicity’), should be taken intoaccount. Good Li affinity indicates strong binding between Li and thematerial's surface, which is desired for achieving both the uniformmolten Li infusion during synthesis and the low Li nucleation barrier inlater battery cycling. For most of the carbon materials, the relativelyweak binding between Li and the carbon surface leads to their poor Liaffinity, excluding many candidates as desired host materials.Surprisingly, it is found that reduced graphene oxide (rGO) hasextremely high lithiophilicity, which is rather unique amongcarbon-based materials. In the presence of strongly reductive molten Li,GO can be partially reduced to remove most of the electrochemicallyunstable surface functional groups such as the hydroxyl groups (—OH).Nevertheless, the more stable functional groups, such as carbonyl (C═O),alkoxy (C—O) and epoxyl (C—O—C), can still remain, which afford strongLi binding on surface, making rGO a desired backbone for hostingmetallic Li.

Results

Herein, a layered rGO host was designed for metallic Li, which exhibitedwell-defined nanoscale gaps in between the layers. Subsequently, moltenLi can readily infuse into the spacious interlayer gaps and the finalstructure of the Li-rGO composite anodes can be described as theperiodic stacking of layered Li and rGO (as shown in the schematic, FIG.2a ). Notably, this structure has several advantages. First, layered rGOstructure offers a stable scaffold for Li stripping/plating, whichsignificantly mitigates the volume change on the electrode level duringcycling. Secondly, the excellent lithiophilicity of rGO with largesurface area provides uniform Li infusion and deposition in the initialsynthesis and later cycling, respectively. In addition, the very top rGOcap layer provides electrochemically and mechanically stable artificialinterface, which can play a role in stabilizing the SEI formed at theelectrolyte/electrode interface. With the above merits, the layeredLi-rGO films provided highly improved electrochemical performance withuniform deposition, stable cycling and low polarization.

In order to obtain the desired layered structure, two operations,namely, fabricating a layered rGO film with nanoscale interlayer spacingand uniformly infusing Li into the interlayer gaps, should be realized.With densely packed GO film prepared by vacuum filtration as thestarting material (FIG. 2b ), a ‘spark’ reaction is implemented forproducing the uniform nanoscale gaps (FIG. 2a , operation 1). It isobserved that when GO film was partially put into contact with molten Li(about 250° C.), a spark reaction can happen quickly across the wholefilm, expanding the film into a much more porous structure. Thisphenomenon could be explained by the sudden pressure release within theGO layers due to the removal of superheated residual water/surfacefunctional groups and the rapid combustion of hydrogen formed under thereduction condition (FIG. 6). The spark reaction successfully generatedthe desirable nanoscale interlayer spacing for Li, and also partiallyreduced the GO film, where the reactive surface functional groups wereselectively removed. As a consequence, a much more porous and stablescaffold can be obtained for later Li intake and electrochemical cycling(FIG. 3c ).

The other operation is to homogeneously infuse Li into the interlayerspacing (FIG. 2a , operation 2). After gently pressing the sparked rGOfilms to smooth the surface, the films were directly employed for Liinfusion without the need for any surface functionalization. Fast anduniform Li intake can be fulfilled by contacting the edge of the rGOfilms with molten Li. Less than about 1 minute is involved for silveryLi to spread across the whole rGO film (FIG. 2d , about 47 mm ofdiameter). Noticeably, such rapid Li intake into rGO films is ratherunusual when compared with other carbon materials (FIG. 8), which canhardly be wetted by molten Li. This unusual property is believed to bethe synergistic effect of the lithiophilic nature of the sparked rGO andthe capillary force produced by the nanoscale gaps. According tofirst-principles calculations (FIG. 9), the surface groups of rGO, suchas carboxyl (3.080 eV) and alkoxy (2.974 eV) groups, exhibit muchstronger binding energy to Li than the bare graphene counterpart (1.983eV). The strong binding can highly increase the surface Li affinity andthus efficiently drive Li intake. Capillary force is another factor forLi infusion (FIG. 10). With superior lithiophilic surface of rGO,capillary force can lift up the molten Li into rGO interlayers. Sinceshrunk gap dimension can offer stronger capillary force, the nanoscaleinterlayer spacing here provided extremely powerful driving force forthe fast Li intake.

To further investigate the obtained nanostructures and their relatedproperties, characterizations on the films before and after sparkreaction as well as after Li infusion were carried out. FIG. 3a, b showthe low magnification and magnified scanning electron microscope (SEM)images of the pristine GO film, which exhibited the typicaldensely-packed feature. After spark reaction (FIG. 3c, d ),significantly enlarged interlayer spacing can be observed. As shown inFIG. 3d , uniform gaps in the scale of several hundred nanometers can beobtained after spark reaction, which is favorable for later Li infusion.Nitrogen sorption analysis (FIG. 11) confirmed the increased porosityafter spark reaction, where the Brunauer-Emmett-Teller (BET) surfacearea of the film increased from about 8.0 m² g⁻¹ to about 394.3 m² g⁻¹.Once Li was infused to the rGO films, as indicated in FIG. 3e, f , thenanoscale gaps were filled by Li, while the uniform layered structurewas still maintained.

The selective reduction of the GO films by spark reaction can besupported by the Fourier transform infrared spectroscopy (FTIR, FIG. 3g), and the X-ray photoelectron spectroscopy (XPS, FIG. 3h,i ), both ofwhich are sensitive to the material's surface chemistry. As shown inFIG. 3g , before spark reaction, strong peak corresponding to —OHstretching (broad peak beyond about 3000 cm⁻¹) was detected, whichrevealed the presence of a large proportion of —OH groups either in theform of adsorbed water or surface —OH. At the same time, characteristicpeaks of other surface groups such as carbonyl (C═O) (about 1730 cm⁻¹)and epoxyl (C—O—C) (about 1045 cm⁻¹) groups were also observed. Incontrast, after spark reaction, the —OH peak became almost undetectable,which verified the removal of both residual water and surface —OHgroups. However, despite slight shift, the peaks of the other morestable oxygen-containing surface moieties were still retained, whichrendered the sparked rGO with excellent lithiophilicity for efficient Liintake. Consistent with the FTIR result, the survey XPS spectraindicated a significant reduction in the amount of 0 element after sparkreaction (FIG. 12). And as can be observed from the deconvoluted C 1sand O 1s spectra (FIGS. 3h and i ), the peaks corresponding to C—O—Hdropped greatly in intensity while the other surface functional groupsexhibited less decrease, which affirmed the selective removal of —OHgroups.

It is further noted that good electrode flexibility is also desired forbattery manufacturing since both pouch and cylindrical cellconfiguration specify flexibility to some extent for either folding orrolling process. For alternative Li hosts with rigid backbone, it ischallenging to afford foldable features without damaging the electrodes.Due to the flexible nature of rGO and the layered structure whichsignificantly reduced the built-in strain after folding, the as-obtainedlayered Li-rGO electrodes exhibited outstanding flexibility andtwistability as shown in FIG. 3j, k . Notably, the electrodes can stillremain intact after several cycles of folding. In practical points ofview, the good flexibility offers exciting possibilities for fabricatingprocessable composite metallic Li anodes and potentially flexiblepackaging for next-generation Li batteries. Furthermore, by changing thethickness of the initial GO film, the thickness of the resulting Li-rGOelectrode can be readily tuned (FIG. 15).

To study the electrochemical performance of the layered Li-rGO films, acarbonate-based electrolyte, which is the dominant electrolyte forLi-ion batteries, was utilized to conduct the tests. It was reportedthat due to its relatively positive reduction potential and thebrittleness of the as-formed SEI layer, uniform Li deposition, limitedside reaction and stable cycling are especially challenging to realizein carbonate-based electrolyte. However, it is of importance to evaluatethe electrochemical performance in carbonate environment due to itsreduced flammability compared with ether electrolyte and bettercompatibility to the existing battery technology. In this example, dueto the rational design of a Li metal host, improvement inelectrochemical properties can be achieved in addition to dimensionalstability even in a carbonate-based electrolyte.

First, uniform Li stripping/plating behavior can be realized using thelayered Li-rGO composite electrode. To study the surface morphology,layered Li-rGO films with adequate thickness of about 50-100 μm wereassembled into symmetric-cell configuration and cycled. Thestripping/plating capacity was fixed at about 1 mAh cm⁻² with a currentdensity of about 1 mA cm⁻² unless further noted. FIG. 4a-c shows thecross section SEM images of the pristine layered Li-rGO film, the filmafter Li stripping, and after one cycle of stripping/plating,respectively. As shown, when Li was stripped (FIG. 4b ), theinterlayered gaps originally occupied by metallic Li appeared again.This illustrated that the interlayered spacing did not undergonoticeable change in the initial Li infusion and the later Li strippingoperations. After plating Li back, as shown in FIG. 4c , most of theinterlayered spacing was refilled by Li. Although the deposited Liexhibited different morphology compared with the initial stage, nonoticeable dendritic Li can be observed. Such observation is a goodindicator of the much more uniform Li deposition provided by the rGOframeworks. The surface morphology after multiple cycles was alsocharacterized in comparison with the bare Li foil counterpart (FIG. 4d-i). FIG. 4d, e show the SEM images of Li foil surface after 10 cycles,from which mossy, dendritic Li can be observed. In contrast, the layeredLi-rGO surface remained smooth after 10 cycles (FIG. 4f, g ) withoutobservable dendrites. The uniform deposition can be sustained even atincreased current density. At a current density of about 3 mA cm⁻² (FIG.4h, i ), the cycled electrode also maintain a flat surface withoutdendritic Li plated on top.

In order to visualize the Li deposition behavior within the rGO film, insitu transmission electron microscope (TEM) technique was exploitedusing a specialized dual-probe biasing TEM holder (NanofactoryInstrument). FIG. 4j schematically illustrates the in situ TEM device,where a Cu metal wire decorated by sparked rGO film was employed as oneelectrode and a W wire covered by Li as the other. A thin oxide layerformed on the surface of Li can perform as the solid electrolyte in thecell. Both side-view (FIG. 4k ) and top-view (FIG. 4l )characterizations were carried out in the in situ study to obtaincomprehensive information on Li deposition. From the time evolutionside-view images, little thickness change can be observed after Lideposition, which shows the merit of layered rGO as the host to maintainthe dimension. The relatively stable dimension can be further confirmedby the ex situ SEM study after Li stripping and plating, which showedabout 20% of thickness fluctuation (FIG. 17). The injection of Li intothe interlayered rGO spacing was further confirmed by the electronenergy loss spectroscopy (EELS). FIG. 4m shows the EELS spectracollected from the corresponding interlayer spots marked in FIG. 4k .Characteristic Li peaks appeared after Li deposition, which indicatesthe successful Li deposition into the interlayer spacing. The top-viewcharacterization further consolidates the uniform Li deposition. Asshown in FIG. 4l , the rGO sheet exhibited continuous Li infusion withslightly expanded lateral dimension. The even expansion across the wholerGO sheet illustrates the uniform Li plating within the host.

The observed uniform Li deposition behavior can be rationalized by thehighly increased number of nucleation sites and the high extent ofinterfacial protection by rGO layers. On one hand, the existence of alarge number of two dimensional Li interlayers provided tremendouslyenhanced activated surface area for Li deposition. Since reduced energybarrier is involved to deposit Li directly on Li, much more uniform Linucleation on the large Li surface within the layers can be achieved. Onthe other hand, the flexible and mechanically strong rGO layer canperform as the stable artificial electrolyte/electrode interface. Withthe stable rGO interface and the small dimension change, SEI crackingand the resulting ‘hot spot’ for uneven Li deposition can be highlysuppressed.

The demonstrated highly reduced electrode dimension fluctuation anduniform Li deposition with stable interfacial layer afforded stablelong-term cycling performance with reduced polarization (voltagehysteresis). FIG. 5a compares the voltage profile of symmetric layeredLi-rGO electrode cells and the bare Li foil electrode counterpartsthrough over 100 cycles. The current density is fixed at about 1 mA cm⁻²with about 1 mAh cm⁻² of stripping/plating capacity. It is apparent thatthe Li-rGO electrodes exhibited stable cycling voltage profile withsmall hysteresis. In contrast, the Li foil displayed gradual increase inhysteresis over the cycles, with almost 100% higher value after 100cycles. To study the evolution of voltage profile in detail, the 1st,10th, 50th and 100th cycle of layered Li-rGO were further enlarged asthe insets in FIG. 5a . The four plots were drawn in the same scale forfair comparison. For the layered Li-rGO electrodes, flat voltageplateaus at both charging and discharging states can be retainedthroughout the whole cycling process without noticeable increase inhysteresis. This is a significant improvement compared with bare Lielectrodes, which showed fluctuated voltage profiles with consistentlyhigher overpotential at both initial and final stages of eachstripping/plating process (FIG. 19). This supports the notion thatlow-barrier Li nucleation and uniform deposition can be obtained withrGO during cycling.

At an increased current density of about 3 mA cm⁻², stable cyclingbeyond 100 cycles with stable hysteresis can still be attained usinglayered Li-rGO electrodes (FIG. 5b ), while the Li foil counterpartexhibited gradual hysteresis augment. At 92 cycles, sudden voltage dropwas detected for bare Li foil with fluctuated voltage in the latercycles, which could be explained by the internal soft short circuit withLi dendrite penetration. The enlarged voltage profiles of the selectedcycles shown in FIG. 5b further confirm the stable cycling of Li-rGOwith flat voltage plateaus through the cycles at the elevated currentdensity.

The highly reduced polarization and stable cycling can be furthersupported by the electrochemical impedance spectroscopy (EIS) analysis,which was conducted on symmetric cells before cycling (FIG. 20) andafter 10 galvanostatic cycles (FIG. 5c ). The corresponding Nyquistplots are shown where the semicircle at the high frequency range is agood indicator of interfacial resistance at SEI and the charge-transferresistance at Li surface. As can be seen from the figures, bare Li foilcells exhibited consistently higher interfacial resistance. It is notedthat before cycling, Li foils showed an extremely large interfacialresistance of about 650Ω, which was originated from the native oxidelayers formed on the electrodes. After 10 cycles, interfacial resistancedropped to a lower value of about 85Ω, due to the break of the nativeoxide layers as well as the dendritic Li formation which significantlyincreased the surface area. In contrast, the layered Li-rGO electrodeshad relatively constantly low interfacial resistance. Before cycling,significantly lower interfacial resistance of about 30Ω can already beattained and after 10 cycles, the layered Li-rGO electrodes exhibitedlittle resistance drop to about 25Ω. This phenomenon illustrates thatmuch better electrode stability and more favorable Li stripping/platingkinetics can be achieved with the layered Li-rGO electrodes.

Notably, the layered Li-rGO electrode exhibited outstandingelectrochemical properties, and also demonstrated uncompromised specificcapacity. As shown in FIG. 5d , unprecedentedly high capacity of about3390 mAh g⁻¹ (based on the weight of the whole electrode) can beextracted when charged to about 1 V, which is very close to the about3860 mAh g⁻¹ theoretical capacity of pure Li (about 87.8% capacityretention compared with pure Li). The high specific capacity can beattributed to the light-weight and porous nature of the sparked rGOwhere the mass of the framework took up about 7 wt % of the wholeLi-infused electrode. Therefore, the choice of carbon-based hostmaterial offers the exciting possibility to fabricate high performanceLi anodes with reduced compensation on capacity.

In summary, a layered Li-rGO composite electrode with uniform nanoscalelayered structure was developed as a Li metal anode. The spark reactionas well as molten Li infusion driven by the lithiophilic rGO surface andthe nanoscale capillary force were combined to realize the structuredesign. Contrary to the “hostless” cycling of bare Li, the well-definedstructure afforded a reduced electrode dimension change during cycling,and also a much more stable electrolyte/electrode interface. Thestabilized interface together with the highly increased activatedsurface for Li nucleation afforded the uniform deposition of Li withoutnoticeable dendritic Li growth at various current densities. In situ TEMwas further exploited to visualize and confirm the dimensional stabilityand uniform Li deposition. As a result, excellent electrochemicalcycling can be obtained. Low overpotential of about 40 mV and about 80mV with flat plateaus can be maintained for over 100 cycles at thecurrent density of about 1 mA cm⁻² and about 3 mA cm⁻², respectively.Simultaneously, near-theoretical specific capacity of about 3390 mAh g⁻²can still be retained due to the low density and high porosity of thegraphene-based host matrix. The electrode design developed in thisexample provides a methodology for fabricating stable Li anodes to beapplied to the next-generation high energy density and high power Libattery system.

Methods

Electrodes synthesis. Firstly, vacuum filtration with anodic aluminumoxide membrane (AAO, Whatman, with diameter of about 47 mm and pore sizeof about 0.2 μm) as filter was utilized to yield the densely packed GOfilms. The highly concentrated GO solution (about 5 g L⁻¹, GrapheneSupermarket) was commercially available and used as received. Theas-obtained freestanding GO films were dried in a vacuum oven at about60° C. for about 48 hrs and then transferred to an argon-filled glovebox with sub-ppm O₂ level. To perform the spark reaction, Li foil (about99.9%, Alfa Aesar) was first melted in a stainless steel pan on a hotplate at about 250° C. The GO films were then put into contact withmolten Li and the spark reaction occurs within seconds. The sparkreaction can partially reduce the GO films into rGO films. Theas-obtained rGO films were gently pressed between two pieces ofstainless steel plates to obtain smooth surface. The edge of the pressedrGO films were made to touch the molten Li, and Li is infused into therGO films with observed color change from black to silvery. Theas-obtained films can perform as the electrodes once cooled down to roomtemperature.

Characterizations. A FEI XL30 Sirion scanning electron microscope isused for SEM characterizations. Before conducting SEM study of thecycled electrodes, the batteries were first disassembled in the gloveboxand followed by gentle rinse in dioxolane (DOL) to remove residue Lisalts. XPS analysis was obtained on a SSI SProbe XPS spectrometer withAl(Kα) source. Raman spectra were measured on a WITEC Raman spectrometerwith an about 531 nm excitation laser. FTIR spectra were recorded on aNicolet iS50 FT/IR Spectrometer (Thermo Scientific). XRD patterns wererecorded on a PANalytical X'Pert instrument and the Li-rGO electrode wasloaded on a glass slide and covered with Kapton tape during measurementsto avoid direct contact with air. N₂ sorption studies were performed ina Micromeritics ASAP 2020 adsorption apparatus at about 77 K and atpressure up to about 1 bar after the samples were first degassed atabout 180° C. overnight. The Brunauer-Emmett-Teller (BET) surface areawas calculated using the adsorption data in a relative pressure rangingfrom about 0.1 to about 0.3.

In situ TEM observation. The in situ electrochemical experiment wascarried out in an FEI Titan 80-300 environmental TEM at the accelerationvoltage of about 300 kV. Nanofactory Instruments Dual-Probe STM-TEM insitu sample holder was used to demonstrate the lithiation process ofrGO. Li metal was used for the counter electrode in this STM-TEM holder.During transferring Li metal electrode inside TEM, the electrode wasexposed to air for about 5 seconds to create a thin Li₂O layer of about20 nm functioning as a solid electrolyte. A relative bias of about −3.4V was induced between the two electrodes, which caused Li⁺ ions to betransferred to rGO electrode through the solid electrolyte. During theexperiment, the electron beam was spread out to lessen electron beameffects.

Electrochemistry. To study the Li stripping/plating processes, theelectrodes were assembled into 2032-type coin cells (MTI) in a symmetriccell configuration. The electrodes used were either the layered Li-rGOfilms or the freshly-scraped Li foils (about 99.9%, Alfa Aesar). Theelectrolytes employed were either about 1 M lithium hexafluorophosphate(LiPF₆) in about 1:1 ethylene carbonate (EC)/diethyl carbonate (DEC)(BASF Selectilyte LP40) with about 2% vinylene carbonate (VC) asadditive for carbonate-based system study, or about 1 M Lithiumbis(trifluoromethanesulphonyl)imide (LiTFSI) in about 1:1 v/v1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) with about 1 wt % lithiumnitrate for ether-based system. Celgard 2325 (25 μm PP/PE/PP) was usedas the separator. Galvanostatic cycling was conducted either on an Arbin96-channel battery tester or a LAND 8-channel battery tester. Theelectrochemical impedance spectroscopy measurements were carried out ona Biologic VMP3 system.

Example 2

High-Activated-Surface Nanocomposite Matrix as Dendrite-Free, Low EnergyBarrier, and Stable Lithium Metal Anodes

Overview

Rechargeable batteries based on lithium (Li) metal chemistry aredesirable for next generation high-energy-density and high-powerelectrochemical energy storage. Nevertheless, excessive dendriticgrowth, virtually infinite dimension change during repeated cyclestogether with limited power output, severely hinders the practicalapplications. Although progress has been made towards realizingdendrite-free deposition, a versatile solution is still desired. Here,this example sets forth a principle to address the multifacetedproblems. In this example, a Li⁺ conductive matrix is constructed as the“host” for nanoscale metallic lithium domains, which was fulfilled byapplying “overlithiation” technique on metastable phase SiO. Supportedby a rigid matrix, highly constant electrode-level dimension duringrepeated cycling can be achieved. Moreover, the Li⁺ conductive nature ofthe matrix afforded a highly increased activated surface area for Li,which afforded the uniform nucleation and thus dendrite-free deposition.In addition, the proposed Li⁺ conductive nanocomposite electrode (LCNE)exhibited extraordinary low energy barriers and constantly lowoverpotential during stripping/plating cycling in both carbonate andether-based liquid electrolyte systems. By in-depth analysis on thebehaviors of Li foil, such phenomenon can also be rationalized by thevast activated surface area and naturally built-in nanosized lithiumdomains as nucleation sites. Furthermore, Li—S prototype cells withLCNEs were assembled, which demonstrated much higher capacity retentionunder high-power operation compared to the conventional Li foilcounterpart. This all-round improvement on electrochemical performanceindicates the effectiveness of the design methodology for developingsafe and stable metallic Li anodes.

INTRODUCTION

Extensive research has been conducted on developing advanced lithium(Li) battery anode substitutes, including silicon, tin and Li metal, forapplications with high-energy-density specification. Among all thealternatives, Li metal is the most prominent candidate. Its lowestelectrochemical potential (Li⁺/Li=about −3.040 V v.s. standard hydrogenelectrode), highest theoretical capacity (about 3860 mAh g⁻¹) and lightweight nature (about 0.534 g cm-3) have attracted broad scientific andtechnological interest. Nevertheless, despite over forty years ofcontinuous efforts, the real application of Li metal anode was keptstagnant due to several intrinsic challenges.

Dimension change at the electrode level is one of the major concerns. Incontrast to conventional graphite with about 10% volume change duringcycling, Li metal foil is a “hostless” electrode, which in principle hasinfinite volume change during stripping/plating cycle (FIG. 22a ). Onone hand, the enormous dimension change will lead to vast built-instress and floating electrode interface, which can bring internal shortcircuit and potential explosion hazard. On the other hand, solidelectrolyte interphase (SEI) is vulnerable to the moving interface andprone to collapse. Continuous SEI formation during cycling can decomposeelectrolyte and consume Li⁺ and also accumulate ionic insulating layerwhich further blocks the ion transfer. Dendritic and mossy Li growtharisen from uneven electric field and Li⁺ concentration distributionduring cycling is another issue (FIG. 22a ). The formation of dendriticLi can penetrate the separator and cause internal short circuit,inducing the danger of overheating and thermal runaway. During strippingprocess, dendritic morphology of Li can aggravate the formation of deadLi and thus, result in low Coulombic efficiency (CE). Moreover, due tothe limited accessible activated surface of Li foil for Listripping/plating, high power operation of the high-energy-density Li—Sand Li—O₂ batteries is still premature, which highly impedes theirpower-intensive applications.

Since the advent of Li batteries, considerable research has beenconducted to understand Li metal plating/stripping behavior, prevent Lidendrite formation and penetration, as well as improve CE. Solidelectrolyte is developed and proven to be efficacious for mitigatingdendrite penetration. However, further improvement in ionic conductivityand interfacial ion transfer for ambient temperature operation is stilldesired. Developing electrolyte additives is also recognized to beeffective in stabilizing SEI and affording more uniform Li deposition.Recently, the concept of engineering chemically and mechanically stableartificial interface has been proposed. With engineered layers oncurrent collector, Li can be selectively deposited underneath theartificial interface and allow the formation of stable SEI on the top.Despite the above-mentioned progress, part of the existing problems wastargeted. Tremendous electrode dimension change and moderate operationpower still remain unsolved. Practically, alternative approaches capableof addressing the multifaceted problems are desired.

Herein, a Li⁺ conductive nanocomposite electrode (LCNE) is developed asLi metal anodes. In the design, a continuous matrix with Li⁺ conductioncapability is included as the “host” and nanosized metallic Li uniformlyembedded in the matrix is included as the Li source (FIG. 22b ). Thisdesign exhibits multiple advantages to solve the above-mentionedproblems. First, with the matrix as the “host”, highly constantelectrode-level dimension can be obtained. This immobilizes theelectrode interface, and also prevents the size fluctuation and built-instress of the cells. Secondly, with Li⁺ conductive species as thematrix, a high activated surface area Li electrode is obtained, whichsignificantly reduced the effective current density during cycling andin turn, reduced the Li nucleation barrier and afforded much moreuniform nucleation. As a result, dendritic growth of Li can be highlysuppressed while stable cycling with low energy barrier and lowoverpotential can be obtained. Another benefit of the approach is thatthe increased surface area offered better kinetics at increased currentdensity, opening up the opportunities to operate cells at high poweroutput. In this example, prototype Li—S batteries were alsodemonstrated, which showed highly improved rate capability especially athigh rate.

Stable Electrode Dimension and Dendritic Suppression

To approach the above-mentioned design, “overlithiation” of SiO isemployed, which is a concept extended from lithiation. Specifically,rather than lithiating the material with Li according to thestoichiometric ratio, for the overlithiation case, certain extra amountof Li is added into the reaction mixture such that excess metallic Liremains in the reaction product. By screening multiple oxides, theoverlithiation behavior of metastable SiO phase is determined to berather unusual. When molten Li and sub-micron-sized SiO powder weremixed at elevated temperature of about 200° C., the mixture exhibitedviolent reaction. With vigorous stirring, Li₂₂Si₅—Li₂O composite withnano-porosity can be obtained in the form of a matrix, where Li₂₂Si₅ canserve as a Li⁺ conductive component. The extra amount of Li can theninfuse into the nano-pores. When the reaction is complete, dry silverypowders can be obtained even above the melting temperature of Li,indicating that almost all metallic Li was infused into the nano-poresof the matrix.

The complete synthetic route of the LCNEs is shown in FIG. 23a . Theprocedure was carried out in an argon-filled glove box with sub-ppm O₂level. The first operation is to overlithiate SiO with molten Li atabout 200° C. The magnified drawing of the overlithiated powderschematically shows the microstructure of the as-obtained nanocomposite,which is composed of Li₂₂Si₅ and Li₂O as the matrix with nanoscale Liembedded inside. Subsequently, mechanical press was applied to theoverlithiated powder to afford the pellet electrodes. The thickness ofthe electrodes can be varied by controlling the amount of powder for acertain pressing area. (FIG. 28).

FIG. 23 shows a digital camera image of an as-obtained LCNE, whichexhibits silvery color with flat surface. The magnified scanningelectron microscope (SEM) image of the LCNE indicates uniformly coveredsurface by nanosized domains of metallic Li, which accounts for theobserved color. To study the internal porous structure of the matrix,about 8 mAh cm⁻² of Li was stripped from the electrode throughgalvanostatic charging. After Li stripping, the top surface directlyfacing the counter electrode exhibited color change from silvery toblack (FIG. 23c ). The black color may come from Li₂₂Si₅ as well as theantireflection effect of the nanoporous structure. From the SEM image(FIG. 23f ) of the delithiated electrode, sub-micron and nanosizedpores, which were initially occupied by Li, can be observed(representative pores are labeled by arrows in FIG. 23f ). Afterwards,when the stripped amount of about 8 mAh cm⁻² Li was plated back, thesilvery color was recovered as can be seen from FIG. 23d . When thecorresponding surface morphology was characterized (FIG. 23g ), it wasfound that almost all of the pores were refilled by Li with noobservable Li dendrites.

Characterization on the variation of the electrode thickness was alsoperformed to examine the electrode-level dimension stability after Listripping and plating. The thickness of LCNEs under three conditions,including pristine (FIG. 23h ), half-way (about 8 mAh cm⁻²) stripped(FIG. 23i ), and after one stripping/plating cycle (FIG. 23j ), wasmeasured and compared. To ensure that each condition had the sameinitial thickness, the electrodes of the three conditions came from thesame original piece of pellet; namely, a piece of LCNE was equally cutinto four individual segments and three of them were picked for study.As is shown in FIG. 23h-j , the three conditions exhibited almostidentical thickness of about 138 μm. This demonstrates that a highlyconstant electrode dimension during stripping/plating processes can beachieved with the Li⁺ conductive matrix as the “host” for Li.

To compare the surface morphology difference of Li foil and LCNE aftercycling, SEM observation was conducted after 20 galvanostatic cycleswith a current density of about 1 mA cm⁻² and a capacity of about 1 mAhcm⁻². FIG. 24a, b schematically illustrate the electrode morphologydifferences of bare Li electrode and LCNE, respectively. The directionof SEM observation is also denoted in FIG. 24a, b . Thelow-magnification and magnified SEM images of the bare Li surface aftercycling are shown in FIGS. 24c and 24e , respectively. From the lowmagnification image, high surface roughness can be observed, withexcessive growth of dendritic Li all over the top. This was furtherconfirmed by the representative magnified SEM image, which shows looselystacked Li dendrites and nanofilaments. In contrast, the LCNE surfaceafter 20 cycles (FIG. 24d ) remained consistently flat over a largearea. Under high resolution observation (FIG. 24f ), no noticeabledendritic Li was observed. Such stark contrast strongly supports thatthe LCNE is capable of maintaining stable structure and suppressingdendrite propagation through continuous cycling.

The above experiments support the uniform Li deposition within thematrix, which can be attributed to the Li⁺ conductive nature and thushighly increased activated surface of the matrix. For bare Li foil, justthe very top surface can have access to Li⁺. As a result, the localelectric field and ion flux will be too concentrated to afford uniformLi growth. In contrast, with Li⁺ conductive matrix, Li⁺ can betransported into the bulk rather than just the top surface, whichactivates much more surface area for Li nucleation. With highlyincreased number of Li nucleus for further deposition, uneven depositionand thus dendritic growth will be highly suppressed.

Low-Barrier, Low-Overpotential and Stable Li Stripping/Plating

To study the electrochemical behavior of the as-obtained LCNEs,symmetric cells with LCNEs as both electrodes were first assembled andtested in carbonate electrolyte. Corresponding cells with symmetric Lifoil electrodes were used as comparison. Firstly, the initialgalvanostatic cycle of both LCNEs and Li foil electrodes was studied. Asshown in FIG. 25a , huge overpotential was observed for Li foil cellsduring the initial stage of charging (A, about 400 mV). As currentpassed through continuously, a gradual drop in overpotential can be seen(gradual decline to about 100 mV). At the discharge stage, overpotentialbump was initially shown (B), followed by a plateau with relatively lowoverpotential. Later, discharge overpotential ascended again (C). Thehigh overpotential and the significant overpotential fluctuations herecan be attributed to the energy barriers for either stripping or platingLi. FIG. 25c schematically shows the Li stripping/plating in the firstcycles corresponding to the high overpotential region A, B, and C shownin FIG. 25a . The origins of the energy barriers at each stage weremarked as (stripping barrier) and (nucleation barrier) dots on theelectrode surface. For as-assembled cells, the surface of Li foil had anative oxide layer that can block ion transfer. When Li foils werecontacted with electrolyte in the cells, an ionic blocking layer of SEIcan also spontaneously form. On one hand, the ionic blocking layer onthe positive electrode will block the Li stripping underneath. On theother hand, the layer on the negative electrode as well as therelatively flat electrode surface will hamper the nucleation of Li ontop. Both effects contribute to the much higher overpotential (regionA). At the discharge stage, the high overpotential region B isoriginated from the nucleation barrier at the positive electrode. Forthe negative electrode, the freshly deposited Li can be readilystripped, where the new SEI layer is still thin. However, the positiveelectrode exhibits concave surface configuration after Li stripping,and, although the original SEI layer may have broken, it still coversthe electrode together with the newly formed SEI. Thus, nucleationunderneath the layer still involves extra energy compensation. Oncenucleus have formed, Li can deposit continuously on positive electrodewith low hindrance, which accounts for the relatively low overpotentialbetween B and C. Later, once the Li freshly deposited from the lastcharging is exhausted, the stripping of original Li from negativeelectrode beneath the thick SEI is involved. This calls for much higherenergy barrier than stripping freshly deposited Li, which leads to theoverpotential augment. Since the later overpotential increase is due tothe stripping of original Li at the negative electrode, this part ofcapacity illustrates the Li loss during cycling, which can be a strongindicator of the poor CE of the Li foils.

In contrast, the LCNE symmetric cells showed extremely flat plateau atboth charging and discharging stage, with no noticeable “bump” duringcycling. In addition, much lower overpotential than the Li foilcounterparts was obtained. This demonstrates that extremely low energybarrier is involved for both nucleation and stripping process. With Li⁺conductive matrix connecting the whole bulk, Li is able to nucleateanywhere Li⁺ can reach. For nano-sized Li embedded inside, large surfaceof Li with thin or no SEI layer is available for nucleation, while thelarge number of nano-sized Li domains themselves can also act asnucleus. For Li stripping, the highly increased activated surface areaaffords uniform Li dissolution without locally-favored stripping.

Electrochemical impedance spectroscopy (EIS) measurements furthersupport the above conclusion. EIS of LCNE and Li foil cells at bothinitial stage (FIG. 33) and after 10 cycles (FIG. 25b ) was measured. Asis shown, the Li foil cells have constantly much higher interfacialresistance compared to the LCNE cells. It is noted that before cycling,Li foil exhibited extremely large interfacial resistance, which can beattributed to the native oxide layers formed on both electrodes. After10 cycles, interfacial resistance dropped to about one eighth of theoriginal value. This results from the break of native oxide layers aswell as the dendritic Li formation which significantly increases thesurface area. In contrast, LCNE cells had constantly low resistance.Before cycling, the LCNE cells already exhibit significantly lowerinterfacial resistance (about 10Ω versus about 650Ω of Li foil) and,after 10 cycles, LCNE cells showed little resistance drop (about 6Ωversus about 85Ω of Li foil). This difference illustrates the much lowerLi stripping/plating energy barriers and polarization of LCNE cells.

Noticeably, for the LCNEs, low overpotential and flat voltage plateauscan be well maintained during repeated stripping/plating of at least 100cycles (FIG. 25d ). In contrast, the Li foil control cells showedgradual increase in overpotential. For the 100th cycle, overpotential ofthe Li foil cells became considerably large. This can be attributed totwo reasons, namely, the accumulation of thick SEI, and the continuousconsumption of electrolyte by decomposition which highly increased theelectrolyte resistance.

The rate capability test of the LCNEs also displays constantly loweroverpotential at different current density. As is shown in FIG. 26a ,where the current density is varied from about 0.5 mA cm⁻² to about 5 mAcm⁻², low overpotential from about 18 mV to about 100 mV can be obtained(with about 18 mV at about 0.5 mA cm⁻², about 28 mV at about 1 mA cm⁻²,about 64 mV at about 3 mA cm⁻² and about 100 mV at about 5 mA cm⁻²) forLCNEs, which is much lower than that of the corresponding Li foilsamples. The LCNEs also exhibited highly improved cycling stability atdifferent current density of about 1 mA cm⁻², about 3 mA cm⁻², and about5 mA cm⁻² (FIG. 26b-d , about 1 mAh cm⁻² stripping/plating capacity). Atabout 1 mA cm⁻² (FIG. 26b ), stable cycling with low overpotential ofabout 32 mV and flat voltage plateaus was observed through 100galvanostatic cycles, while the control Li foil samples showedcontinuous increase in overpotential from about 65 mV to about 140 mV.The cycling stability of the LCNEs and the Li foil electrodes divergedeven more at higher current densities (FIG. 26c,d ). At currentdensities of about 3 mA cm⁻² and about 5 mA cm⁻², LCNE continued toafford good cycling stability with low overpotential, while in contrast,the Li foil counterparts yielded dramatic overpotential increase overcycles. After tens of cycles, it is noted that the overpotential of Lifoil electrodes started to decrease and fluctuate, possibly due to theinternal short circuit caused by Li dendrite.

The cycling stability study at various current densities was furthercarried out in ether-based electrolyte. Ether-based electrolyte withsmall amount of LiNO₃ as additives can be a better electrolyte for Limetal anodes since a stable soft SEI layer can form to accommodate thesurface fluctuation during stripping/plating. As a result, much moreuniform Li deposition can be achieved with Li foil. Nevertheless, theLCNEs can still outperform the Li foils in ether-based system. As shownin FIG. 34, at a current density of about 1 mA cm⁻², the Li foilelectrode showed high overpotential in the early cycles followed bygradual decrease. This decreasing phenomenon can be attributed to thedendritic Li growth, which significantly increases the surface area. ForLCNE cells, constantly lower overpotential of about 15 mV can beobtained through the whole 100 cycles. At high current density of about5 mA cm⁻², LCNEs also exhibited stable cycling while the Li foilcounterpart yielded noticeable voltage fluctuation.

Improved Rate Capability of Li—S Batteries

Metallic Li is the most promising anode material for Li—S batterysystems. With the dendrite-free, low energy barrier and stable LCNE asanodes, the electrochemical performance is demonstrated in Li—Sbatteries. Here, about 12.5 μL cm⁻² of about 5 M Li₂S₈ in about 1:1 v/vDOL/DME (about 2 mg cm⁻² equivalent mass loading of S) is loaded on KOHtreated electrospun carbon nanofibers as the cathodes. Rate capabilitytest with current densities varied from about 0.2 C to about 2 C wascarried out (based on theoretical capacity of S of about 1673 mAh g⁻¹, 1C=about 3.34 mA cm⁻²). FIG. 27 shows the notable rate capabilitydifference between Li—S cells with LCNEs and Li foils as the anodes. Atearly stage with low current density, high capacity retention about 1000mAh g⁻¹ can be achieved for both anodes. However, it is shown in FIGS.27a and 27b that the cell with LCNE as the anode exhibited loweroverpotential compared to the Li foil counterpart. More pronounceddifference appeared when the current density was further increased. Atabout 2 C, which is about 6.69 mA cm⁻², the Li foil-S cell lost itssecond plateau (Li₂S₆→Li₂S) with cut-off voltage at about 1.7 V, whichgave rise to significant capacity drop. In contrast, the LCNE-S cellyielded much improved kinetics, with a second plateau of about 1.9 Veven at the high rate of about 2 C. FIG. 27c shows the capacityretention at different C-rates during cycling. For LCNE-S cell, about1050 mAh g⁻¹ of specific capacity can be obtained at about 0.2 C, whileat ten times higher current density (about 2 C), the capacity stillremained over about 600 mAh g⁻¹. Much lower specific capacity retentionwas observed for the Li foil-S cell especially at high rate, for thecapacity dropped significantly from about 950 mAh g⁻¹ at about 0.2 C toless than about 100 mAh g⁻¹ at about 2 C.

SUMMARY

In conclusion, this example sets forth the development of a Li⁺conductive matrix as the “host” for Li metal anodes, which was fulfilledby the overlithiation of SiO to yield uniformly mixed Li—Li₂₂Si₅—Li₂Onanocomposite LCNE. The LCNE provides dendrite-free Li depositionbehavior with highly constant electrode-level dimension. Due to thehighly increased activated surface achieved by Li⁺ conductive network,much lower stripping/plating energy barrier and thus stable cycling withlow overpotential, flat plateau can be attained. By systematicallyanalyzing the mechanism of energy barriers at differentcharging/discharging stage from Li foil electrodes, the origin of theoutstanding electrochemical performance in LCNEs can be understood. Inaddition, further demonstration is made of the excellent rateperformance at different current densities of about 0.5 mA cm⁻², about 1mA cm⁻², about 3 mA cm⁻², and about 5 mA cm⁻², where consistent lowoverpotential of about 18 mV, about 28 mV, about 64 mV and about 100 mVcan be obtained, respectively. Stable cycling stability can also beobtained for at least 100 cycles at various current densities. Even inether-based electrolyte system, enhanced electrochemical performance canstill be attained. By introducing the high performance LCNEs into Li—Sbatteries, improved rate capability especially at high current densitycan be observed. This example sets forth a methodology for Li metalanode design and opens opportunities for the next-generationhigh-energy-density, high-power Li metal batteries under safe and stableoperation.

Methods

Overlithiation of SiO. Before performing overlithiation of SiO, SiOpowders (Aldrich, 325 mesh) was first ball-milled for about 5 hrs withhigh energy mechanical milling machine to yield sub-micron sizes. Theas-obtained SiO powders were then dried in vacuum oven with temperaturefixed at about 60° C. for about 48 hrs. The fully dried powders weretransferred into an argon-filled glove box with sub-ppm 02 level forlater overlithiation process. To overlithiate the SiO, about 0.5 g ofSiO powders were firstly put into a stainless steel crucible. Thecrucible was then heated on a hot plate at about 200° C. Afterwards, Lifoil (about 99.9%, Alfa Aesar) was weighted and put into the crucible.The mass of Li foil can be varied based on the desired amount ofextra-Li. The additional amount of Li is controlled <about 1.0 g toyield uniformly-mixed dried powders. After Li was melted, the mixturewas vigorously stirred for homogeneous reaction. Once reaction complete,the as-obtained dried powders were cooled in the argon atmosphere toroom temperature.

LCNE fabrication. After the overlithiated composite powders wereattained, mechanical press was applied to fabricate the LCNEs. To pressthe as-obtained powders into the electrodes, certain amount of powderswere weighted and poured into the pellet die (Aldrich) following byabout 10 tons of pressure. The thickness of the LCNEs can be controlledby the amount of powders pressed. All processes were finished in theglove box.

Characterizations. SEM images were obtained with a FEI XL30 Sirionscanning electron microscope. For the SEM characterizations on batteriesafter cycling, the electrodes were first extracted from the coin cellsin the glovebox and followed by gentle rinse in DOL to remove Li saltresidue. XRD patterns were recorded on a PANalytical X'Pert instrument.To protect the high reactive Li related compound from the air, the XRDsamples were loaded on a glass slide and covered with Kapton tape inglove box prior to the XRD measurements.

Electrochemical measurements on symmetric cells. To study the Listripping/plating processes within a symmetric cell configuration,2032-type coin cells (MTI) were assembled. The electrodes used wereeither the above-mentioned LCNEs or Li foils. The electrolytes employedwere either about 1 M lithium hexafluorophosphate (LiPF₆) in about 1:1ethylene carbonate (EC)/diethyl carbonate (DEC) (BASF Selectilyte LP40)with about 10% fluoroethylene carbonate (FEC, Novolyte TechnologiesInc.) and about 1% vinylene carbonate (VC, Novolyte Technologies Inc.)as additive for carbonate-based system study, or about 1 M Lithiumbis(trifluoromethanesulphonyl)imide (LiTFSI, Aldrich) in about 1:1 v/v1,3-dioxolane (DOL, Sigma-Aldrich)/1,2-dimethoxyethane (DME,Sigma-Aldrich) with about 1 wt % lithium nitrate (Aldrich, 99.99%) forether-based system. Celgard 2325 (25 μm PP/PE/PP) was used as theseparator. The control bare Li cells were assembled using freshlyscraped Li foil. Galvanostatic cycling was conducted either on an Arbin96-channel battery tester, or a LAND 8-channel battery tester. Theelectrochemical impedance spectroscopy measurements were carried out ona Biologic VMP3 system.

Lithium polysulfide (Li₂S₈) solution preparation. For the preparation ofabout 5 M Li₂S₈ solution, about 0.56 g of sulfur and about 0.115 g ofLi₂S were weighted and added into about 4 mL of about 1:1 v/v DOL/DMEsolution mixed solution with the addition of lithium nitrate additive(about 5 wt. %). The obtained suspension was stirred and heated at about80° C. overnight to yield red-brown Li₂S₈ solution.

Li—S batteries tests. To study the electrochemical performance of LCNEsin Li—S batteries, 2032-type coin cells (MTI) were assembled. Thecathodes used about 1 cm² KOH treated electrospun carbon nanofibers toadsorb the Li₂S₈ catholyte. The detailed procedures for carbon nanofiberfabrication is included in the supplementary information. About 12.5 μLof above-synthesized 5 M Li₂S₈ solution (about 2 mg equivalent massloading of sulfur) was uniformly drop-cast onto the carbon fiber as thecatholyte. The electrolyte was freshly prepared by dissolving about 1 MLiTFSI in about 1:1 v/v DOL/DME containing LiNO₃ (about 1 wt. %).Electrolyte in each cell was fixed to be about 20 μL. Celgard 2325 (25μm PP/PE/PP) was used as the separator. The rate capability test of thecells was carried out using an Arbin 96-channel battery tester.

Supplementary Information

Fabrication of electrospun carbon nanofibers. To fabricate the carbonnanofibers, about 1.0 g of polyacrylonitrile (PAN, Aldrich, M_(v) ofabout 150,000) was first dissolved in about 10 mL dimethylformamide(DMF) and vigorously stirred at about 80° C. to yield an uniformlydissolved viscous solution. The freshly prepared solution was thenfilled into a syringe for electrospinning. The electrospinning wasperformed with about 15 kV of static electric voltage and about 10 cm ofair gap distance. The flow rate of solution was set at about 0.25 mLh⁻¹. A carbon paper (8 cm×8 cm) was used as the collection substrate.After electrospinning for about 3 hrs, the collected nanofiber on thecarbon paper was peeled off to yield a freestanding mat. The mat waspre-carbonized by heating up to about 300° C. at a rate of about 1° C.min⁻¹ in a box furnace and held for about 4 hrs. Afterwards, thefreestanding pre-carbonized PAN nanofiber films were further carbonizedunder an argon atmosphere at about 850° C. (heating rate of about 3° C.min⁻¹) for about 4 hrs to generate the carbon nanofiber films (CNFs).

KOH treatment on CNFs. About 20 wt % of potassium hydroxide (KOH,Aldrich) was first dissolved in about 1:1 v/v deionized water (DIwater)/isopropyl alcohol (IPA) solution. The carbon nanofiber obtainedfrom the above-mentioned method was then dipped into the solution forabout 5 seconds. The residue solution on the CNFs was absorbed by tissuepaper. This process is repeated for 5 times. Afterwards, the CNFs weredried in a vacuum oven at the temperature of about 60° C. After fullydried, the CNFs were annealed in argon atmosphere at about 700° C. forabout 2 hrs. The as-obtained CNFs were then punched into about 1.0 cm²disk with average weight of 2.3±0.1 mg per piece.

Example 3

Lithium Coated Polymeric Matrix as Reduced Volume Change andDendrite-Free Lithium Metal Anode

Overview

Lithium metal is a desired anode material for the next-generationhigh-energy-density batteries. Nevertheless, dendrite growth, severeside reactions and virtually infinite relative volume change duringcycling have impeded practical applications. Here, this exampledemonstrates a metallic Li anode design by directly infusing molten Liinto a polymeric matrix. The electrospun polyimide fiber employed wasstable against the high temperature, highly reactive molten Li, and viaa conformal layer of ZnO coating to render the fiber surfacelithiophilic, molten Li can be readily drawn into the matrix, affordinga nanoporous Li electrode. Of note, the polymeric backbone afforded anuniform Li stripping/plating behavior from the underlying conducting Li,which effectively confined Li within the electrode matrix duringcycling, realizing reduced volume change at the whole electrode leveland effective dendrite suppression. Moreover, the porous electrodereduced the effective current density per unit area and therefore, flatvoltage profiles and extremely stable cycling of more than 100 cyclescan be achieved even at a high current density of about 5 mA/cm² in bothcarbonate and ether-based electrolyte. The advantages of the polymericmatrix and porous electrode provide important insights into the designprinciples of stable Li metal anodes.

INTRODUCTION

The ever-increasing demand for high-energy-density storage systems fortransportation (electric vehicles), portable electronics and otherapplications has stimulated intensive research on rechargeable batteriesthat go beyond the conventional lithium (Li) ion chemistry. Among thepossible options, Li metal is the most desired anode material due to itshighest theoretical capacity (about 3860 mAh/g) as well as the lowestelectrochemical potential (about −3.040 V vs standard hydrogenelectrode). Despite the appealing properties, Li metal electrode hasbeen plagued for decades with the problem of ramified growth duringrepeated stripping/plating and the associated electrolyte decomposition,which lead to serious safety concerns and poor battery cyclingefficiency.

Li is highly reactive such that, in a liquid electrolyte, it reactsspontaneously with solvent molecules and salt anions to form aninsoluble layer of solid electrolyte interphase (SEI). When SEI becomesstabilized to block electron transfer, this passivating film can slowdown or ideally prevent electrolyte from further decomposition.Nevertheless, as a “hostless” electrode, the Li metal anode has avirtually infinite relative volume change during stripping/plating,resulting in the mechanical instability of the SEI layer and theformation of cracks. The cracks expose fresh Li underneath and locallyenhance the Li ion flux, leading to non-homogeneous Li growth (dendrite,filament, and so forth) that can induce internal short circuit andthermal runaway with potential safety hazard. Moreover, the high surfacearea, dendritic Li growth brings about a continuous loss of both workingLi and electrolyte (recurrent SEI formation), which gives rise to lowCoulombic efficiency (CE) and rapid capacity decay.

Research on the Li metal stripping/plating process has deepened theunderstanding but has failed to solve the above-mentioned problems in aneffective manner. On one hand, the use of solid electrolytes to suppressdendrite propagation remains premature at current stage, for they oftenfall short of meeting the high power specification at ambienttemperature due to limited ionic conductivity, together with issues suchas large interfacial impedance. On the other hand, other approaches todendrite mitigation in liquid electrolyte focus on the stabilization ofSEI via adjusting the electrolyte composition and additives. Thougheffective, most additives will be continuously consumed during batterycycling so that the suppression effect is not fully sustainable.Alternatively, the application of a mechanically stable artificial SEIcoating such as polymer or solid-state blocking layers has beenproposed. For example, a nanoscale interfacial engineering approachbased on interconnected hollow carbon nanospheres, ultrathintwo-dimensional boron nitride or oxidized polyacrylonitrile fibers canbe used to control the dendrite growth and improve the cycling CE.Nevertheless, these approaches adopted the galvanostatic Liplating/stripping approach on a current collector, which still areunable to address the issue of virtually infinite volume change sincethe electrodes expand during Li plating and shrink during stripping. Inaddition, contrary to Li-ion batteries where Li ions are stored inprelithiated cathodes, many of the intensively studiedhigh-energy-density battery chemistries (e.g., Li-air and Li—S) involvecathodes in the non-lithiated form. Therefore, it is apparent that ametallic Li anode design with little volume change at the wholeelectrode scale and long-term cycling stability in a liquid electrolyteis of particular importance.

Herein, this example demonstrates a rational design of a metallic Lianode that successfully achieved a reduced volume change at the wholeelectrode level and stable, dendrite-free Li cycling. Several importantdesign principles were employed. Firstly, in order to realize reducedvolume change, a chemically as well as electrochemically stable matrixis included to sustain a highly constant electrode volume duringcycling. In addition, complete confinement of Li within the matrix isdesired to preserve a constant electrode dimension, and, therefore, thedirect nucleation of Li on the top surface of the matrix should bereduced or prevented. Moreover, a porous electrode is desirable since areduced local current density is beneficial to alleviate dendritepropagation. Following the above-mentioned rationale, a Li-coatedpolyimide (PI) matrix design for metallic Li anode is proposed. Theelectrospun polymeric fibers afforded a chemically and electrochemicallyinert matrix, which is favorable to confine the stripping/plating of Lisolely or primarily within the matrix. Notably, PI is rather unusual asa high performance polymer that exhibits excellent chemical stability,heat resistance and mechanical strength above the melting point of Li(about 180° C.). However, the wetting of molten Li on PI polymer ispoor. By applying a layer of ZnO coating via atomic layer deposition(ALD) on the PI fibers, molten Li can react with ZnO and subsequentlyinfuse into the PI matrix, resulting in a free-standing, currentcollector-free Li electrode. More importantly, the conducting functionis separated from the matrix, where infused metallic Li itself served asthe sole or primary electron transport media. As a result, theelectrically insulating surface after Li stripping effectively preventedthe direct plating of Li on the top surface of the matrix in thesubsequent cycle, bringing about a well-confined, dendrite-free Listripping/plating behavior that successfully addressed the problem ofinfinite volume change. Moreover, the obtained electrode is highlyporous so that the reduced effective current density resulted in flatvoltage profiles and stable cycling of at least 100 cycles in bothcarbonate and ether-based electrolytes even at a high current density ofabout 5 mA/cm², which stands in stark contrast to the fluctuated andunstable cycling profile of bare Li foil electrodes.

Results

Fabrication of the Li-coated PI electrode. FIG. 37 illustrates thefabrication process of the Li-coated PI matrix electrode. Anelectrospinning method is employed to obtain the PI fiber matrix.Thermogravimetric analysis (TGA, FIG. 42) confirmed that the electrospunPI fiber is stable up to about 450° C., which is well above the meltingpoint of Li (about 180° C.). Such high heat-resistance ensures that thematrix can withstand the temperature of molten Li in order to fabricatethe metallic Li anode. Nevertheless, molten Li cannot adequately wet thebare PI matrix (FIG. 43). Due to the high surface tension of molten Lion PI fiber, large driving force is involved for Li to infuse into thematrix. Rather than physical absorption, surface chemical reaction whichcan afford much higher driving force is desired. Through screening onmaterials that can undergo conversion reaction with Li, it is determinedthat a layer of conformal ZnO coating applied to the matrix via atomiclayer deposition (ALD) can render the matrix wettable by molten lithium,or “lithiophilic”. Subsequently, when the core-shell PI—ZnO matrix wasplaced in contact with molten Li, ZnO reacted with molten Li and extraLi can be drawn into the matrix, affording a Li-coated PI electrode.

Characterization of the Li-coated PI electrode. The morphology of theelectrospun PI fibers was characterized using a scanning electronmicroscope (SEM). As can be seen from FIG. 38a , the fibers werecontinuous and uniform in general with a diameter of about 400 nm. AfterALD coating, the surface of the fibers roughened due to the accumulationof a conformal layer of ZnO nanoparticles (FIG. 38b ). Evident PI—ZnOcore-shell structure can be observed from the cross-sectional SEM image(FIG. 38c ), where the PI core appeared darker in color and thethickness of the ZnO shell was measured to be about 30 nm (FIG. 44).Scanning transmission electron microscope (STEM) energy dispersive X-ray(EDX) elemental mapping (FIG. 38d ) as well as line scan (FIG. 45)resolved the distribution of C (from the PI backbone) and Zn (from theZnO coating), further confirming the core-shell structure of the fibermatrix after ALD. FIG. 38e shows the SEM top view of the PI matrix afterLi coating. It appears that Li was drawn into the matrix preferentiallyalong the fibers, and thus, the matrix was not densely coated with Li.The porous nature of the resulting matrix can be further revealed fromthe cross sectional SEM image (FIG. 46), where noticeable pores can beobserved. X-ray diffraction (XRD) was employed to understand thecompositional evolution of the matrix (FIG. 38f ). As can be seen fromthe XRD spectra, the ZnO layer reacted with molten Li to form LiZn alloyand Li₂O during the Li coating process. Since the electron percolationpathway of the alloy particles within the non-conducting Li₂O matrix isgenerally long, the matrix remained low in electrical conductivitycompared to metallic Li after Li coating. Such a porous and electricallyinsulating matrix renders favorable electrochemical features to theresulting Li electrode, which will be elaborated in detail in latersections.

It is important to ensure the stability of the polymeric backbone incontact with the highly reductive molten Li. Therefore, Fouriertransform infrared spectroscopy (FTIR) was employed (FIG. 38g , insetshows the general chemical structure of PI), where the existence of PIcan be identified from three characteristic peaks corresponding toasymmetric C═O stretching, symmetric C═O stretching and C—N stretching.After ALD and the subsequent contact with molten Li, the transmittanceintensity was reduced due to the coating layers, while the threecharacteristic peaks remained, confirming the intact polymeric matrix.In addition, neither anodic nor cathodic decomposition peaks can beobserved from the cyclic voltammetry (CV) of the pristine PI (FIG. 47),indicating the chemical stability of the polymeric matrix towardselectrochemical cycling.

The capacity of the Li electrode was determined to be above about 2000mAh/g based on the weight of the whole composite electrode via Listripping (FIG. 48). Thus, the existence of the matrix did notsignificantly compromise the high specific capacity of the Li anode.Noticeably, by adjusting the thickness of the electrospun matrix, whichcan be done by changing the electrospinning time, the thickness of thefinal Li electrode can be tuned accordingly to match the capacity of thebattery cathode (FIG. 49).

Well-confined Li stripping/plating within the matrix. Investigation wasmade of the stripping/plating process of the Li-coated PI matrix using atwo-electrode symmetric cell configuration assembled in 2032 coin cellswith carbonate-based electrolyte (about 1 M lithium hexafluorophosphate(LiPF₆) in about 1:1 ethylene carbonate, EC/diethyl carbonate, DEC,BASF). Interestingly, the electrode exhibited a well-confinedstripping/plating behavior (FIG. 39). Top fibers of the matrix wereexposed after stripping away about 5 mAh/cm² Li at a current density ofabout 1 mA/cm² (FIG. 39a ), which indicates that the top Li layers weredissolved more favorably during stripping. Subsequently, when about 3mAh/cm² Li was plated, Li was observed to be deposited into the matrixand partially filled the space between the fibers (FIG. 39b ). Finally,when all the stripped Li was plated back (FIG. 39c ), the top surface ofthe matrix was covered again by Li (similar to FIG. 38e ) with nodiscernible dendrites. The well-confined plating behavior can berationalized by the removal of the conductive Li component in the priorstripping process, and thus the exposure of the electrically insulatingPI surface. Since Li plating occurs primarily where electrons meet Liions, the exposed insulating surface was rendered unfavorable for Linucleation. Instead, the metallic Li confined within the matrix servedas the electron conductor such that the deposition of Li occurredprimarily on the underlying reserved Li. In addition, the much largereffective surface area lowered the overall deposition barrier and thuspreventing the formation of “hot spots”. As a result, uneven Lideposition can be suppressed. On the contrary, if the electrons could beefficiently transported to the electrolyte-facing top surface or theelectrodes exhibited limited surface area, undesirable Listripping/plating behavior may occur after recurrent cycles (FIG. 39,top-right scheme). Direct Li nucleation on the top surface might beeasier due to the high availability of both electrons and Li ions, whichprovides favorable sites for dendrite growth while leaving the interiorvoids empty.

Dendrite-free cycling with minimum volume change. The morphology of thesurface of the Li-coated PI matrix was studied after 10 cycles ofgalvanostatic stripping/plating in EC/DEC (FIG. 40a-c ). Due to theabove-mentioned well-confined Li cycling behavior, the surface of theLi-coated PI electrode remained consistently flat even at a high currentdensity of about 5 mA/cm² (note that the uniform fibrous features inFIG. 40a-c are the fiber matrix and shall not be mistaken as Lidendrites). On the contrary, for the bare Li electrodes, rough surfaceand excessive mossy Li growth can be observed after 10 cycles even at arelatively low current density of about 1 mA/cm². Such drasticallydifferent result further demonstrates the merit of the PI matrix ondendrite suppression.

Moreover, due to the existence of the host matrix, the issue of infinitevolume change associated with the “hostless” Li stripping/plating cannow be solved. Even with the complete stripping of Li, the change inelectrode thickness was minimal. For example as shown in FIG. 40f-g ,the electrode was on average about 253 μm before stripping and remainedas about 247 μm after complete stripping, which was merely about 2.4% ofchange (a relatively thick electrode was chosen for the more precisedetermination of thickness variation). However, for bare Li foil, about1 mAh/cm² capacity represents about 4.85 μm thickness of Li (seeSupporting Information for detailed calculation). Therefore, at leasttens of microns of electrode thickness fluctuation can be expected forone single layer Li electrode in a commercial cell. Considering theconventionally applied stacking or rolling battery configuration withmultiple layers, the accumulated dimension fluctuation can betremendous. It is noted that for later cycles, due to the formation ofdendritic Li and thus a porous electrode, the dimension fluctuation canbe even larger. Thus, it is apparent that the existence of a stablematrix and the well-confined Li cycling behavior are desirable toalleviate the electrode level volume change, addressing the potentialsafety concerns.

Electrochemical cycling stability. The galvanostatic cycling performanceof the Li-coated PI matrix was studied in both carbonate (EC/DEC) andether (about 1 M Lithium bis(trifluoromethanesulfonyl)imide, LiTFSI inabout 1:1 1,3-dioxolane, DOL/1,2-dimethoxyethane, DME with about 1 wt %lithium nitrate) based electrolyte and compared with bare Li electrode(FIG. 41, FIGS. 50-52). At a current density of about 1 mA/cm² in EC/DEC(FIG. 41a ), the symmetrical cell of bare Li exhibited a large Listripping/plating overpotential (>about 100 mV vs Li⁺/Li), whichincreased considerably within the first 100 cycles (>about 170 mV in the100th cycle). In contrast, the Li-coated PI matrix showed a much loweroverpotential (about 35 mV in the initial cycle) but also achieved verystable cycling for at least 100 cycles (about 40 mV overpotential in the100th cycle). The difference in cycling stability (FIG. 41b-c ) andoverpotential (FIG. 41d ) became increasingly pronounced at highercurrent densities. The stripping/plating overpotential for Li-coated PImatrix was about 70 mV and about 110 mV at a current density of about 3mA/cm² and about 5 mA/cm² respectively and the values remainedsubstantially constant within 100 cycles. However, the bare Li electrodesuccumbed to substantial voltage fluctuation at just 88 cycles at acurrent density of about 3 mA/cm² and 75 cycles at a current density ofabout 5 mA/cm² which might be attributed to possible dendrite-inducedsoft short circuit. Similarly, in DOL/DME electrolyte, the Li-coated PImatrix again outperformed the bare Li electrode (FIG. 50). Noticeably,although DOL can improve the cycling life of Li metal anodes due to theformation a relatively flexible oligomer SEI, the bare Li electrodestill exhibited a necking behavior (overpotential first decreases thenincreases) during cycling, which is a characteristic sign for dendriteformation in early stage and SEI accumulation later. Nevertheless, theLi-coated PI matrix maintained flat, substantially constant cyclingprofiles and reduced overpotential at all current densities. Suchexceptional long-term cycling performance is a good indicator of thesuperior CE and more uniform Li deposition/dissolution of the Li-coatedPI electrode.

DISCUSSION

Two factors contributed to the excellent electrochemical performance ofthe Li-coated PI matrix, namely, the porous nature of the electrode andthe non-conducting nature of the exposed matrix surface after Listripping. As mentioned previously, molten Li was drawn into the matrixpreferentially along the fibers during the Li coating operation,resulting in a porous Li electrode. The high porosity can be furtherconfirmed by the fast electrolyte uptake during cell assembly (FIG. 51).Such high porosity increased the surface area of the electrode, whichcan in turn significantly reduce the effective current density duringcycling. As a result, the Li stripping/plating overpotential was muchsmaller for the Li-coated PI matrix, especially at high currentdensities (FIG. 41d ). Correspondingly, the electrochemical impedancespectroscopy revealed a much reduced interfacial charge transferresistance for the Li-coated PI matrix compare to bare Li (about 10times lower in EC/DEC, FIG. 41e and FIG. 52). Further, the reducedeffective current density rendered the Li-coated PI matrix a flatstripping/plating voltage profile (FIG. 41f-g and FIG. 53). For the bareLi electrode, large “overpotential bumps” at the beginning and the endof each stripping or plating process can be observed, especially for theearly cycles (FIG. 54). This phenomenon can be explained by the highspecific kinetic hindrance for non-uniform Li dissolution/deposition athigh current density. The fluctuation attenuated in later cycles due tothe formation of mossy Li, which increased the surface area of theelectrode, reducing the effective current density. However, whencompared to the fluctuated profile of the bare Li electrode, the voltageprofile of Li-coated PI was flat at all current densities throughout thecycling, demonstrating the advantages of the porous electrode. Inaddition, the effective current density during Li stripping/plating hasan impact on the dendrite formation and growth. Lower effective currentdensity results in reduced electrolyte decomposition and the related SEIformation during cycling so as to suppress the dendrite growth.Therefore, the porous electrode structure, in addition to thenon-conducting polymeric matrix, which led to the well-confined Lidissolution/deposition behavior, effectively ensured the dendrite-freecycling of the Li-coated PI electrode, giving rise to stable long-termperformance.

In conclusion, this example demonstrates a free-standing, porousmetallic Li anode by infusing molten Li into a core-shell PI—ZnO matrix.The excellent heat resistance and chemical stability of the PI fibersafforded the structural integrity of the matrix during Li coating andthe subsequent battery cycling, while the conformal ALD ZnO coatingprovided the driving force for the molten Li infusion. Noticeably, theexposed non-conducting fibers after Li stripping reduced the directplating of Li on the top surface of the electrode, which effectivelyconfined Li within the matrix. In this manner, dendrite-free and reducedvolume change Li stripping/plating can be successfully achieved,addressing the biggest concerns for Li metal anode. Remarkably,different from the dense bare Li electrode, the large porosity of theLi-coated PI electrode considerably decreased the effective currentdensity during cycling. As a result, flat voltage profiles and long-termcycling stability can be realized even at a high current density ofabout 5 mA/cm². The benefits of non-conducting polymeric matrix andelectrode porosity shed new light on the design principles of metallicLi anodes and open new opportunities to the realization of thenext-generation high-energy-density battery systems based on Li metalchemistries.

Methods

Electrospinning of the PI matrix. About 15 wt % PI powder (DuPontCP-0650) was dissolved in N-methyl-2-pyrrolidone (NMP) and stirred atabout 750 rpm in an about 60° C. oil bath overnight to afford ahomogeneous solution. Subsequently, the solution was loaded into a glasssyringe with a stainless steel needle tip, which is connected to avoltage supply (ES30P-5W, Gamma High Voltage Research). The appliedpotential on the needle was about 15 kV, the distance between the needletip and the graphite paper collector was about 15 cm and the pumpingrate was about 10 μL/min. In addition, a negative voltage of about 1 kVwas applied at the collector to improve the homogeneity of theelectrospun film. The thickness of the electrospun PI matrix can betuned by adjusting the electrospinning time.

Atomic layer deposition. The conformal ZnO layer was coated on the PIfibers via atomic layer deposition using the Cambridge Nanotech Savannah5100 at about 80° C. with diethyl zinc (DEZn) and de-ionized (DI) wateras precursors. The pulse times for DEZn and DI water were about 15 mswith about 60 s waiting between each pulse. Approximately 300 cycleswere included to obtain a ZnO coating with desirable Li wettingproperty.

Li coating of the core-shell PI—ZnO matrix. The Li coating process wascarried out in an Argon-filled glovebox with sub-ppm 02 level. In atypical process, freshly scraped Li foil (about 99.9%, Alfa Aesar) wasput into a stainless still crucible and heated to melt on a hotplate(VWR). Subsequently, the edge of the core-shell PI—ZnO matrix (punchedinto about 1 cm² disks) was put into contact with the molten Li. Li cansteadily rise up and wet the whole matrix, affording the final Lielectrode.

Electrochemical measurements. The processes of Li stripping/plating werestudied using a symmetric cell configuration by assembling theelectrodes into 2032-type coin cells. The electrolytes employed wereeither about 1 M lithium hexafluorophosphate (LiPF₆) in about 1:1ethylene carbonate (EC)/diethyl carbonate (DEC) (BASF Selectilyte LP40)or about 1 M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) inabout 1:1 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) with about 1 wt.% lithium nitrate additive. The separator used was Celgard 2325 (25 μmPP/PE/PP). The control bare Li cells were assembled using freshlyscraped Li foil. Galvanostatic cycling was conducted on a standardbattery tester (Wuhan LAND electronics Co., Ltd.). A substantiallyconstant current was applied to the electrodes during repeatedstripping/plating while the potential was recorded over time. Theimpedance measurements were carried out using a Biologic VMP3multichannel system.

Characterization. SEM images were taken using a FEI XL30 Sirion scanningelectron microscope at an acceleration voltage of about 5 kV. In orderto observe the surface morphology of lithium after cycling, theelectrodes were disassembled from the coil cell in the glovebox followedby gentle rinse in DOL. STEM images, the corresponding EDX elementalmapping and line scan were obtained on a FEI Tecnai G2 F20 X-TWIN. XRDpatterns were recorded on a PANalytical X'Pert instrument. The Lielectrode was loaded on a glass slide and covered with Kapton tapeduring XRD measurements to avoid direct contact with air. FTIR spectrawere recorded on a Nicolet iS50 FT/IR Spectrometer (Thermo Scientific).TGA was performed on a TA Instrument Q500 TGA in air atmosphere with aheating rate of about 5° C./min.

Supplementary Information

Calculation of the change in electrode dimension duringstripping/plating for bare Li.

Theoretical specific capacity of Li=about 3860 mAh/g

Density of Li=about 0.534 g/cm³

Volumetric capacity of Li=Theoretical specific capacity of Li×Density ofLi=about 2061.24 mAh/cm³

Thickness of Li corresponding to 1 mAh/cm² capacity=1 mAh/cm²/Volumetriccapacity corresponding to about 4.85 μm

Example 4

Lithium/Carbon Composite Anode: Melt Infusion of Lithium into aThree-Dimensional Conducting Scaffold with “Lithiophilic” Coating

Overview

Lithium metal based battery is considered to be a particularly desirableenergy storage system due to its high theoretical capacity and low anodepotential. However, dendritic growth and virtually infinite relativevolume change during long-term cycling often lead to severe safetyhazards and catastrophic failure. Here, a lithium-scaffold compositeelectrode is developed by lithium melt-infusion into a three-dimensional(3D) porous carbon matrix with “lithiophilic” coating. Lithium isuniformly entrapped on the matrix surface and in the 3D structure. Theresulting composite electrode possesses a high conductive surface areaand excellent structural stability upon galvanostatic cycling. Stablecycling of this composite electrode is demonstrated with small Liplating/stripping overpotential (<about 90 mV) at a high current densityof about 3 mA/cm² over 80 cycles.

INTRODUCTION

Nowadays the increasing demand for portable electronic devices as wellas electric vehicles raises an urgent need for high energy densitybatteries. Lithium (Li) metal anode is regarded as the “Holy Grail” ofbattery technologies, due to its light weight (about 0.53 g/cm³), lowestanode potential (about −3.04 V vs. the standard hydrogen electrode), andhigh specific capacity (about 3860 mAh/g vs. about 372 mAh/g forgraphite anode). It possesses an even higher theoretical capacity thanthe recently intensely researched anodes such as Ge, Sn and Si. Inaddition, the demand for copper current collectors (about 9 g/cm³) inconventional batteries with graphite anodes can be eliminated byemployment of Li metal anodes, hence reducing the total cell weightdramatically. Therefore, Li metal could be a favorable candidate to beutilized in highly promising, next-generation energy storage systemssuch as Li-Sulfur (Li—S) battery and Li-Air battery.

The safety hazard associated with Li metal batteries, originating fromthe uncontrolled dendrite formation, has become a hurdle against thepractical realization of Li metal based batteries. The sharp Lifilaments can pierce through the separator with increasing cycle time,thus provoking internal short-circuiting. Attempts to settle thisbottleneck focus on solid electrolyte interphase (SEI)stabilization/modification by introducing various additives. Theseelectrolyte additives interact with Li quickly and create a protectivelayer on the Li metal surface, which helps reinforce the SEI.Furthermore, the employment of interconnected hollow carbon spheres andhexagonal boron nitride (h-BN) as mechanically and chemically stableartificial SEI can effectively block Li dendrite growth.

In addition to the notorious Li dendrite formation, another factor thatcontributes to the battery short-circuiting is the volume change of Limetal during electrochemical cycling. During battery cycling, Li metalis deposited/stripped without a host material. Thus the whole electrodesuffers from a drastic volume change (infinity in a relative scale)compared with the finite volume expansion of other anodes for lithiumion batteries such as Si (about 400%) and graphite (about 10%). As aresult, the mechanical instability induced by the virtually infinitevolumetric change would enhance the risk of separator penetrationaccording to unstable surface and failure of SEI. However, littleattention has been placed on the volume fluctuation problem of the“hostless” Li. This example proposes that a host scaffold to trap Limetal inside can effectively reduce the volume change of the wholeelectrode and therefore maintain the electrode surface.

Herein, this example sets forth a Li—scaffold composite anode and itseffectiveness on addressing the safety issue of traditional “hostless”Li metal electrode. The scaffold serves as a rigid host with Liuniformly confined inside to accommodate the infinite volume change ofLi metal during cycling. In order to create the composite electrode asshown in FIG. 55A, it is desired to find a suitable porous material tohost the Li metal. A desirable scaffold for Li encapsulation should havethe following attributes: (1) mechanical and chemical stability towardselectrochemical cycling; (2) low gravimetric density to achieve highenergy density of the composite anode; (3) good electrical and ionicconductivity to provide unblocked electron/ion pathway, affording fastelectron/ion transport; and (4) relatively large surface area for Lideposition, lowering the effective electrode current density and thepossibility of dendrite formation. By considering these aspects, carbonbased porous materials are chosen. Specifically, electrospun carbonfiber network was utilized as an example material to illustrate thecapability of this composite anode to sustain the volume fluctuation andshape change during each electrochemical cycle.

The encapsulation of Li metal inside the porous carbon scaffold presentsas a major challenge. Compared with other battery electrode materialswhich can be fabricated via various synthetic processes, manufacturingof Li metal based microstructures and nanostructures are difficult dueto the high reactivity of Li. The lack of spatial control of depositionand unsmooth Li surface due to dendritic Li formation can impede Liencapsulation by entrapping Li through electrochemical deposition.Therefore, development of versatile and scalable approaches forencapsulating Li inside porous carbon or other scaffold to create Libased composite electrodes is highly desired.

Li metal possesses a low melting point of about 180° C., and it wouldliquefy into molten Li under anaerobic atmosphere when heated up to itsmelting point. A strategy is developed as follows: melt-infusion ofmolten Li into a “lithiophilic” matrix, which has low contact angle withliquid Li. A porous material with a thin layer of a lithiophilic coatinghas excellent wettability with liquefied Li and thus could function asthe host scaffold for Li entrapment. In this example, theafore-mentioned electrospun carbon fiber network modified withlithiophilic coating—silicon (Si), was employed as the scaffold for Liencapsulation. Li readily and quickly flows into the fiber layer regionand occupy the empty spaces between fibers. The resulting compositestructure, denoted as Li/C, remains both mechanically and chemicallystable under galvanostatic cycling; moreover, it provides a stableelectrode/electrolyte interface. The effective anode current densitycould also be reduced due to an enlarged surface area for Li nucleationprocess, which in turn causes superior electrochemical performancesunder the same test conditions. To summarize, in contrary to the“hostless” Li metal, the as-proposed Li/C composite anode is able toaccommodate the volume variation and therefore mitigate the potentialsafety hazard; moreover, the reduced current density, rooted to largersurface area, also triggers a greatly improved electrochemicalperformance, with stable cycling of about 2300 mAh/g for more than 80cycles at a high current density of about 3 mA/cm².

Results and Discussion

Design and fabrication of Li/C composite anode.

A Li—carbon nanofiber scaffold composite electrode is developed whichpossesses a stable volume during cycling. The carbon nanofiber scaffold,with an average diameter of about 196 nm (FIG. 60), was prepared bycarbonization of the oxidized polyacrylonitrile (PAN) fiber. Theschematic representation of this Li—carbon nanofiber scaffold design isshown in FIG. 55A, where a noteworthy operation in the fabricationprocess is Li entrapment. In this design, Li was heated above itsmelting temperature under argon atmosphere, and the resulting molten Liwas absorbed into the scaffold. A thin layer of Si was coated onto thescaffold surface by chemical vapor deposition (CVD) to assist thismelt-infusion process. To investigate the effectiveness of Si coating onLi wettability of a wide variety of porous materials, molten Li dropletwas placed on several different porous materials, including copper foamand carbon fiber network. As shown in FIG. 55B, the molten Li droplettends to ball up and avoid contact with surfaces without anymodification, indicating an unfavorable wettability. This “lithiophobic”effect possibly originates from the lack of bonding interaction betweensurface (carbon or Cu) and molten Li. For surface modified objects, theSi coating reacts with molten Li to create a binary alloy phase—lithiumsilicide with some bonding interactions to pure Li. This reaction drivesand guides molten Li to wet the entire surface and fill in the porousstructure. Therefore the Si layer by CVD functions as a “lithiophilic”coating promoting good wetting property of liquefied Li. FIG. 55C is asequence of the time-lapse images of the Li melt-infusion process. Forunmodified carbon framework, molten Li could not wet its surface. Incomparison, Si-coated carbon framework shows good wettability as moltenLi quickly flows into the structure under capillary force.

Characterizations of Li/C Composite Anode.

Scanning electron microscope (SEM) and transmission electron microscope(TEM) characterizations were performed to study the morphology of thecarbon fiber as well as the spatial distribution of the Si coating. FIG.56A shows the bare carbon fiber with a smooth surface, indicating theabsence of coatings. The diameter of the carbon fiber is about 200 nm(FIG. 60). In contrast, Si-coated carbon fiber displays a rough edgewith Si nanoparticles homogeneously deposited on the fiber surface (FIG.56B). These Si nanoparticles, with an average size of severalnanometers, stack together to form a continuous, dense shell on thecarbon fiber core. FIG. 56C is the typical TEM image of the Si layerunder higher magnification. The Si shell by CVD could be observed with adifferent contrast (lighter region) and has a thickness of about 30 nm.The linear scan spectra again confirms the formation of a uniform Sicoating on the carbon fiber surface (FIG. 56D). To further verify thedistribution of Si inside the entire fiber layer instead of each singlefiber, energy-dispersive X-ray spectroscopy (EDS) mappings of the fiberlayer from top and cross section were performed. FIGS. 56F, 56G, 56I,and 56J exhibit carbon and silicon signals, revealing the presence ofthe Si nanoparticles throughout the whole fiber layer region. Moreover,as illustrated in FIGS. 56F and 56I, Si is uniformly coated over theentire fiber layer. Therefore, this carbon fiber framework with Sihomogeneously distributed could serve as a desirable matrix for Li meltinfusion and ensure a conformal Li entrapment.

After Li infiltration, the resulting Li/C composite displays a smoothsurface with metallic luster compared with the pristine carbon fibernetwork, indicating that Li is uniformly confined inside the matrix(FIGS. 57A and 57B). SEM characterization was conducted on the carbonfiber before (FIGS. 57C and 57D) and after (FIGS. 57E and 57F) Liinfusion. As illustrated in FIG. 57C, the carbon fiber network exhibitsa 3D porous structure with interconnected fibrous morphology.Furthermore, it provides a high surface area of about 15.6 m²/g (byBrunauer-Emmett-Teller method based on nitrogen adsorption) as well asempty inner spaces among fibers for efficient Li storage. The fiberlayer has an uniform thickness of about 100-120 μm (FIG. 57D). The insetimage in FIG. 57D indicates an interconnected network which provides acontinuous conductive pathway for facile ion/electron transport. Thecarbon fiber film was subjected to a current-voltage (I-V) measurementand the calculated through-plane resistance based on the I-V curve isabout 67 ohm for a disc of about 1 cm² and about 100-120 thick (FIG.61). The average sheet resistance was also determined to be about 4200ohm/sq using a four point meter (FIG. 62). The good electricalconductivity of this PAN-based carbon fiber network ensures a facileelectron transport of the 3D backbone. FIGS. 57E and 57F present themorphology of the Li/C composite material from top and cross section,respectively. It is observed that the carbon fiber network is uniformlycoated by metallic Li and the interspace pores between fibers werefilled with Li. The entrapped Li is restrained within the fiber layer asno Li is observed protruding beyond the fiber mat boundary. From thecross section view, it is also concluded that the absorbed Li forms aporous structure with internal voids. X-ray diffraction (XRD) pattern ofthe Li/C reveals two phases existing in the composite (FIG. 57G). Theidentified peaks are indexed as Li₂₁Si₅ (PDF #01-079-5589) and Li (PDF#00-015-0401), confirming the entrapment of Li inside the scaffold. Theabsence of graphitic carbon signals confirms the amorphous phase of theelectrospun carbon fiber. Amorphous carbon is inert to metallic lithiumand provides excellent chemical/mechanical stability towardselectrochemical cycling. Signal of lithium silicide (Li₂₁Si₅) is alsoobserved due to the reaction of Si coating with the molten Li.

Electrochemical Performances of Li/C Composite Anode.

To evaluate the structural stability of the Li/C electrode duringgalvanostatic cycling, symmetrical coin cells (2032-type) with twoidentical Li/C electrodes were assembled. To standardize the test,control cells were fabricated using bare Li anode with equal thickness(about 120 μm). The cells were subjected to a typicalLi-stripping-plating process with current density of about 1 mA/cm². Afixed amount of Li was stripped from anode and then deposited back. Theanode volume change could be visually pronounced by stripping a total ofabout 10 mAh/cm² Li, which is about half of the anode capacity (about50% extraction). The Li/C anode shows a more stable voltage plateaucompared with bare Li anodes, implying a superior interfacial stabilityduring cycling (FIG. 58A). In addition, the Li/C anode has much lowervoltage hysteresis. The corresponding electrode thickness change duringvarious stages of cycling is shown in FIG. 58B. The thickness of bare Lielectrode is halved to about 60 μm after about 50% extraction of the Li,indicating a large volume change. Whereas thickness of Li/C compositeremain largely unaltered (from about 116 μm to about 110 μm and back toabout 116 μm) upon cycling, which again confirms the effectiveness ofthe Li/C composite anode in maintaining a highly constant thickness.Electrode thickness was analyzed based on a summary of 15 differentlocations for each sample (FIG. 63).

The SEM images of electrode cross sections during various stages ofcycling are presented as FIG. 58C-H. The shape and thickness of thecontrol Li metal electrode is expected to change upon every cycle owingto the lack of a host scaffold (FIG. 58C-E). The Li/C compositeelectrode experiences a small volume change, with an intact and stablesurface (FIG. 58F-H). The inset images in FIG. 58G and FIG. 58H indicatethat the fibrous morphology is preserved after Li cycling. The carbonfiber network serves as a rigid backbone for Li to reversibly depositinto/extract from, thus ensuring an unchanged shape of the wholeelectrode as well as a stable interface between the electrode andelectrolyte.

Morphologies of the bare Li electrode and Li/C electrode after repeatedLi-plating/stripping cycles were also analyzed by SEM. Cross-sectionalimage of the cycled Li electrode shows an uneven surface (FIG. 58I)while the cross section of the cycled Li/C electrode exhibits an intactelectrode shape with stable interface between electrode and electrolyte(FIG. 58J). It should be noted that this rough surface of Li electrodeafter cycling originates mainly from the repeated inhomogeneous and“hostless” Li deposition/stripping upon cycling, inducing a noticeablemechanical pressure on the separator. Hence the risk of battery failureis greatly enhanced. Top view of the cycled Li electrode shows Lidendrites protruding upwards (FIGS. 64A and 64B), again confirming theovergrowth of Li filaments. For Li/C electrode, the absence ofnoticeable large Li dendrite is ascribed to the effectiveness of thehigh-surface-area 3D matrix on reducing the actual current density (FIG.64C). Therefore the formation of dendritic Li could be suppressed tosome extent. However, visible mossy Li is still present (FIG. 64D).

To investigate the electrochemical performances of the Li/C anode,symmetrical cells were assembled with Li/C and bare Li (control),respectively. About 1 M lithium hexafluorophosphate (LiPF₆) inco-solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) wasused as electrolyte. Cells with bare Li electrodes and Li/C electrodeswere compared in terms of plating/stripping voltage profiles (FIG.59A-D), electrochemical impedance (FIG. 59E), rate performance (FIG.59F), and cycling behavior (FIG. 59G).

FIG. 59A-D depicts voltage profiles of a typical Li-plating/strippingprocess at various test conditions (about 0.5 mA/cm² and about 3 mA/cm²for about 0.5 mAh/cm² and about 1 mAh/cm²). These voltages curves forbare Li electrode consistently show significant dips and bumps whereasthose for Li/C electrode exhibit flat and stable plateau. It is thusconcluded that Li incorporation/extraction for Li/C is initiated andmaintained much more readily compared to bare Li electrode. Since Li isplated/stripped on/from Li itself, nucleation barrier originating fromlattice mismatch is reduced. Therefore, the unstable voltage curves forbare Li anodes can be ascribed to the variation of electrode surface andshape over time. AC impedance test was performed to examine theinterfacial resistance in a symmetrical cell configuration. Themeasurement was conducted on both pristine electrodes and those after1st cycle of Li plating/stripping in order to eliminate the influence ofsurface impurities. The interfacial impedance for pristine Li and Li/C,as indicated by the semicircle at high frequency, stay at about 450 ohmand about 60 ohm, respectively (FIG. 59E). For Li and Li/C electrodeafter first cycle, the interfacial impedance reach about 90 ohm andabout 40 ohm, respectively (FIG. 59E). The larger impedance for thecontrol Li indicates that the non-uniform and “hostless” Liplating/stripping has resulted in poor Li ion transport. The reduced Liion transport resistance for Li/C electrode is in accordance with thesmaller cycling overpotential and more stable voltage curve in FIG.59A-D.

FIG. 59F presents the rate behavior of the Li/C composite. Voltagehysteresis is plotted versus cycle numbers under various currentdensities ranging from about 0.5 C to about 10 C. Voltage hysteresis isspecified as the sum of overpotential for Li deposition and Lidissolution. As illustrated in FIG. 59F, Li/C electrode delivers asteadily increasing voltage hysteresis of about 40 mV, about 70 mV,about 190 mV, about 300 mV, and about 380 mV at about 0.5 C, about 1 C,about 3 C, about 5 C and about 10 C, respectively. Meanwhile, bare Lielectrode delivers a much larger voltage polarization of about 150 mV,about 180 mV, about 330 mV, about 550 mV and about 870 mV under sametest conditions. This good rate capability of Li/C reveals facileion/electron transport, owning to the existence of unblockedion/electron conductive pathway inside the Li/C composite. FIG. 59Gshows the long term cycling profile of the symmetrical coin cells. ForLi/C electrode, it can cycle stably for more than 80 cycles under a highcurrent rate of about 3 mA/cm². The voltage plateau is stable at eachcharge-discharge process. Even after 80 cycles, the cell shows noevidence of dendrite-induced failure. Whereas for bare Li electrode, thevoltage hysteresis gradually increases with increasing cycle time untilan abrupt drop (FIG. 59G). This sudden voltage change could be caused byexcessive formation and continuously building up of SEI, followed by thebreaking down of separator. It is evident that cells with “hostless”bare Li electrode exhibit a predominantly and irregularly fluctuatedvoltage profile which is consistent with the cell failure by adendrite-induced short-circuit.

CONCLUSION

In summary, a melt-infusion approach is used to effectively encapsulateLi inside a porous host scaffold. The infiltrated Li uniformly confinedwithin the host matrix creates a Li composite material. It can deliver ahigh gravimetric capacity of about 2300 mAh/g as stable anodes for Limetal batteries. This design affords remarkable battery performance witha low interfacial impedance, stable Li plating/stripping voltage profileand long cycle life, due to its high conductive surface area, stableelectrolyte/electrode interface and negligible volume fluctuation.Compared with “hostless” Li metal electrode, this Li/C compositeelectrode has multiple advantages and therefore can open a new avenuefor solving the intrinsic problems of Li metal based batteries.

Methods

Si-coated carbon nanofiber fabrication. Polyacrylonitrile (PAN),polyvinylpyrrolidone (PVP) and dimethylformamide (DMF) used werecommercially available from Sigma-Aldrich Chemical Corporation. About0.5 g PAN (M_(w)=about 150,000) and about 0.5 g PVP (M_(w)=about1,300,000) were added into about 10 ml DMF. The as-prepared solution wasstirred vigorously at about 80° C. for about 6 hours (about 380 rpm).Afterwards the solution was electrospun into nanofibers with thefollowing electrospinning parameters: about 18 cm of nozzle-to-collectordistance, about 15 kV of electrospinning voltage, about 0.3 ml/h of pumprate, and about 9 cm×9 cm of the graphite paper collector size. Afterabout 40 hours of electrospinning, the as-prepared fiber mat wasstabilized in air at about 300° C. for about 2 hours in a box furnace(Lindberg/Blue M, Thermo Electron Corporation). The oxidized fiber wasthen transferred to tube furnace (Thermo Electron Corporation) to becarbonized at about 700° C. under argon atmosphere for about 3 hourswith a heating rate of about 5° C./min. Si was coated onto carbon fibernetwork via chemical vapor deposition (CVD) with the followingparameters: about 100 sccm of silane flow rate, about 30 ton ofpressure, and about 490° C. for about 30 min.

Li infiltration. Surface of the Li metal foil was polished to remove theimpurities. For Li melt-infusion process, Li was heated over about 300°C. on a nickel sheet under argon atmosphere. The oxygen level is keptbelow about 0.1 ppm to ensure little oxide on the molten Li surface.Different porous materials were dipped into the molten Li and held untilLi flow into the structure substantially completely.

Characterization. SEM study and elemental mapping was conducted using anFEI XL30 Sirion scanning electron microscope with a field emission gun(FEG) source. TEM characterization and linear scan was performed with anFEI Tecnai G2 F20 X-TWIN transmission electron microscope. XRD wascarried out using X-ray diffractometer (X'Pert Pro, PANalaytcal) with CuKα radiation. Cycled coin cells were dissembled inside an argon-filledglove box and washed with EC before further measurements. Specificsurface area of the carbon nanofiber was determined byBrunauer-Emmett-Teller (BET) method based on nitrogen gas adsorption,using a Micromeritics ASAP 2020 analyzer. The samples (about 100 mg intotal) were degassed at about 150° C. for about 24 hours beforeanalysis.

Electrochemical measurements. Li metal sheet and Li/C composite were cutinto disks of about 1 cm² by a punch machine (MTI). Symmetrical MTItype-2032 coin cells were assembled with two identical electrodes insidean argon-filled glove box (MB-200B, Mbraun). About 1 M LiPF₆ in EC/DEC(about 1:1 vol. %) was used as electrolyte. Battery testing was carriedout with a 96-channel battery tester (Arbin Instruments).Electrochemical impedance was probed at room temperature over thefrequency from about 0.1 Hz to about 200 kHz on an electrochemicalworkstation (BioLogic Science Instruments, VMP3). To perform thecurrent-voltage measurement, carbon fiber film (about 1 cm², about100-120 μm thick) was sandwiched between two copper foils connecting tothe electrochemical station (BioLogic Science Instruments, VMP3). Scanrate was about 50 mV/s, within the range of about −2 to about 2 V. Sheetresistance of the carbon fiber film was measured by four point probetechnique, using a four point meter (Rchek, Model #RC2175). The sampleswere cut into proper size (rectangle, about 1 cm×3 cm, thickness ofabout 100-120 μm) and the average sheet resistance is based on 20samples.

Calculation of gravimetric specific capacity of the Li/C electrode.Gravimetric specific capacity of Li metal anode is about 3860 mAh/g. Theweight percentage of Li in the composite is about 60% (Table 1), and thecorresponding gravimetric specific capacity is about (3860mAh/g×60%)=about 2316 mAh/g.

Supplementary Information

TABLE 1 Weight of the bare carbon fiber, Si-coated carbon fiber and Liinfiltrated carbon fiber. Average Materials Weight (mg) Bare carbonfiber 3 Si coated carbon fiber 3.8 Si coated carbon fiber 9.3 with Liinside

The carbon fiber mat with thickness of about 100 μm was cut into smalldisks before the subsequent measurements. The weight of the material ismeasured based on a disk of about 1 cm² for 10 samples. Weightpercentage of Li in the Li/C composite is about 60%.

As used herein, the singular terms “a,” “an,” and “the” may includeplural referents unless the context clearly dictates otherwise. Thus,for example, reference to an object may include multiple objects unlessthe context clearly dictates otherwise.

As used herein, the terms “substantially,” “substantial,” and “about”are used to describe and account for small variations. When used inconjunction with an event or circumstance, the terms can refer toinstances in which the event or circumstance occurs precisely as well asinstances in which the event or circumstance occurs to a closeapproximation. When used in conjunction with a numerical value, theterms can refer to a range of variation of less than or equal to ±10% ofthat numerical value, such as less than or equal to ±5%, less than orequal to ±4%, less than or equal to ±3%, less than or equal to ±2%, lessthan or equal to ±1%, less than or equal to ±0.5%, less than or equal to±0.1%, or less than or equal to ±0.05%.

Additionally, amounts, ratios, and other numerical values are sometimespresented herein in a range format. It is to be understood that suchrange format is used for convenience and brevity and should beunderstood flexibly to include numerical values explicitly specified aslimits of a range, but also to include all individual numerical valuesor sub-ranges encompassed within that range as if each numerical valueand sub-range is explicitly specified. For example, a ratio in the rangeof about 1 to about 200 should be understood to include the explicitlyrecited limits of about 1 and about 200, but also to include individualratios such as about 2, about 3, and about 4, and sub-ranges such asabout 10 to about 50, about 20 to about 100, and so forth.

While the disclosure has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the disclosure asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, operation or operations, to the objective, spirit and scope ofthe disclosure. All such modifications are intended to be within thescope of the claims appended hereto. In particular, while certainmethods may have been described with reference to particular operationsperformed in a particular order, it will be understood that theseoperations may be combined, sub-divided, or re-ordered to form anequivalent method without departing from the teachings of thedisclosure. Accordingly, unless specifically indicated herein, the orderand grouping of the operations is not a limitation of the disclosure.

What is claimed is:
 1. A method of manufacturing a composite lithiummetal anode, comprising: providing a porous matrix; providing liquefiedlithium metal; and infusing the liquefied lithium metal into the porousmatrix.
 2. The method of claim 1, wherein the porous matrix includescarbon or a carbon-based material.
 3. The method of claim 1, wherein theporous matrix includes a Li⁺ conductive material.
 4. The method of claim1, wherein the porous matrix has a surface area of at least about 15 m²g⁻¹.
 5. The method of claim 1, wherein the porous matrix has a densityof about 3 g cm⁻³ or less.
 6. The method of claim 1, wherein providingthe porous matrix includes applying a coating to a porous material, andthe coating has an affinity for the liquefied lithium metal.
 7. Themethod of claim 6, wherein applying the coating includes applying acoating of M, wherein M is a Group 14 element.
 8. The method of claim 7,wherein M is silicon.
 9. The method of claim 6, wherein applying thecoating includes applying a coating of an oxide of M′, wherein M′ is ametal different from Li.
 10. The method of claim 9, wherein M′ is atransition metal.
 11. The method of claim 9, wherein M′ is zinc.